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Majority of three (2-out-of-3 voting): truth table, K-map and NAND form

Two-out-of-three voting, (A AND B) OR (A AND C) OR (B AND C): the eight-row table, a Karnaugh map with three overlapping groups, the normal forms and the 9-gate NAND construction.

Three voters, A, B and C, each say yes or no, and the output is yes when at least two agree. The formula lists the three possible pairs and ORs them; the engine reads it as majority being true exactly when at least one of the pairs A AND B, A AND C or B AND C is wholly true. On the eight-row table that happens on four rows, Σm(3, 5, 6, 7): the three rows with exactly two yes votes and the row with three.

The function is not a tautology, counterexample A = FALSE, B = FALSE, C = FALSE, and it is satisfiable, witness A = FALSE, B = TRUE, C = TRUE. The simplifier found no law that shortens the expression; it stays at five operators, and the DNF converter returns it unchanged. The CNF, through distribution twice, absorption twice and idempotence once, is (A OR B) AND (A OR C) AND (B OR C). The Karnaugh map draws three two-cell groups, A AND B, A AND C and B AND C, all sharing cell 7, three prime implicants, and derives the original. Gate counts: 9 NAND, 39 NOR.

Majority is the voter in triple-modular-redundant hardware, the quorum rule in small committees and a common carry function: the full-adder carry page shows the same truth table reached from a different formula. In code it often appears disguised as a count, votes >= 2, which is fine until someone adds a fourth voter and the threshold stops meaning majority. The engine writes the count as one gate, AT_LEAST(2; A, B, C), and the counting-constraint expansion law is the rewriting that turns it into the three pairs. The three overlapping groups on the map explain why no term can be dropped: every pair covers the all-true cell 7, but each also owns a cell no other pair reaches.

majority := (A AND B) OR (A AND C) OR (B AND C) · output

Exact checks

Always true (tautology)? No Counterexample: A = F, B = F, C = F · Exhaustively checked and solver cross-checked · 1 rows
Always false (contradiction)? No Counterexample: A = F, B = T, C = T · Exhaustively checked and solver cross-checked · 4 rows
Can be true? Yes Example assignment: A = F, B = T, C = T · Exhaustively checked and solver cross-checked · 4 rows

Truth table (8 rows)

Majority of three (2-out-of-3 voting): truth table, K-map and NAND form
#ABCmajority
0FALSEFALSEFALSEFALSE
1FALSEFALSETRUEFALSE
2FALSETRUEFALSEFALSE
3FALSETRUETRUETRUE
4TRUEFALSEFALSEFALSE
5TRUEFALSETRUETRUE
6TRUETRUEFALSETRUE
7TRUETRUETRUETRUE

Plain-language reading

Beginnermajority is true exactly when at least one of the following holds (several may hold):
1. both A and B are true
2. both A and C are true
3. both B and C are true
In every other case, majority is false.
About the operators:
- OR is inclusive: it is true when at least one part is true, including when several are.
- AND is true only when all of its parts are true.
Intermediatemajority is true exactly when at least one of the following holds (several may hold):
1. both A and B are true
2. both A and C are true
3. both B and C are true
In every other case, majority is false.
Advancedmajority is true exactly when at least one of the following holds (several may hold) [inclusive disjunction ∨]:
1. both A and B are true [conjunction ∧]
2. both A and C are true [conjunction ∧]
3. both B and C are true [conjunction ∧]
In every other case, majority is false.
Formal notation: (A ∧ B) ∨ (A ∧ C) ∨ (B ∧ C)

Logic diagram

Majority of three (2-out-of-3 voting): truth table, K-map and NAND form Majority of three (2-out-of-3 voting): truth table, K-map and NAND form A B C majority OR AND AND AND
Majority of three (2-out-of-3 voting): truth table, K-map and NAND form Majority of three (2-out-of-3 voting): truth table, K-map and NAND form A B C majority OR AND AND AND

Simplification, step by step

The engine found no law that shortens this expression; it is already as short as its rewriting laws can make it.

(A AND B) OR (A AND C) OR (B AND C)

Karnaugh map

A \ BC
00
01
11
10
0
0m0
0m1
1m3
0m2
1
0m4
1m5
1m7
1m6
  1. cells m6, m7A AND B
  2. cells m5, m7A AND C
  3. cells m3, m7B AND C
(A AND B) OR (A AND C) OR (B AND C) · Derived (A AND B) OR (A AND C) OR (B AND C). Exactly equivalent on every assignment.

Minterms and maxterms

majority(A, B, C) = Σm(3, 5, 6, 7)majority(A, B, C) = ΠM(0, 1, 2, 4)

Variable order: A, B, C · 8 rows

Normal forms

Conjunctive normal form (CNF)

(A OR B) AND (A OR C) AND (B OR C)

3 clauses, 6 literals · complete · Distribution ×2, Absorption ×2, Idempotence ×1

Disjunctive normal form (DNF)

(A AND B) OR (A AND C) OR (B AND C)

3 terms, 6 literals · complete

NAND-only and NOR-only

NAND-only form

NAND(NAND(NAND(NAND(A, B), NAND(A, C)), NAND(NAND(A, B), NAND(A, C))), NAND(B, C))

9 gates · verified equivalent on every row

NOR-only form

NOR(NOR(NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C)))), NOR(NOR(B, B), NOR(C, C))), NOR(NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C)))), NOR(NOR(B, B), NOR(C, C))))

39 gates · verified equivalent on every row

The same rule as code

Python
# -*- coding: utf-8 -*-
# Generated by Tangibool (tangibool.python/1). Do not edit by hand; regenerate from the source revision.
# Model: not stated
# Source revision: reference
# Interpretation status: not stated
"""Tangibool evaluator (decision model).

Generated deterministically from the validated Boolean model; it evaluates exactly that
model. It does not establish that the model is a complete, legal, or safe statement of a
real-world rule, and evaluating examples does not verify the rule.

Source revision: reference
Interpretation status: not stated
Exporter: 1.0.0 (format tangibool.python/1; engine 1.0.0; semantics tangibool-semantics/1)
Logic digest: b7cf2f66ad74a4d23184d37a24dcffbc8278c9909728a0c4872e824905513fe8
Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78

Inputs (dict keys, declared order) and generated parameter names:
    'A' -> input_A
    'B' -> input_B
    'C' -> input_C

Outputs (declared order) and their functions:
    'majority' -> output_majority(inputs)

Assumptions (the declared scope, checked by in_scope(inputs)):
    none; every complete assignment is in scope.

Input contract:
    Every required input must be exactly True or False (type(value) is bool). Missing
    inputs, None, strings, integers (including 0 and 1), and keys that are not declared
    inputs raise InputError before anything is evaluated, including inputs that a
    short-circuiting operator would never read. Results are always bool.

Scope:
    Decision functions raise OutOfScopeError when the inputs violate an assumption; an
    out-of-scope assignment is never turned into a False result. Pass check_scope=False
    to evaluate without the scope check, and use in_scope(inputs) to test it separately.

Usage:

    import tangibool_model as model

    model.evaluate({'A': False, 'B': False, 'C': False})
    # -> {'majority': False}
    model.output_majority({'A': False, 'B': False, 'C': False})
    # -> False
    model.evaluate({'A': 1, 'B': False, 'C': False})
    # raises InputError: 1 is an int, not a bool
"""

import collections.abc as _abc
from typing import Dict, Mapping, Tuple

__all__ = [
    "MODEL_INFO",
    "IDENTIFIER_MAP",
    "INPUT_NAMES",
    "OUTPUT_NAMES",
    "ASSUMPTIONS",
    "InputError",
    "OutOfScopeError",
    "minterm_index",
    "in_scope",
    "evaluate",
    "output_majority",
]

MODEL_INFO = {
    "format": "tangibool.python/1",
    "mode": "decision",
    "model": None,
    "revision": "reference",
    "interpretation": None,
    "exporter": "1.0.0",
    "engine_version": "1.0.0",
    "semantics_version": "tangibool-semantics/1",
    "logic_digest": "b7cf2f66ad74a4d23184d37a24dcffbc8278c9909728a0c4872e824905513fe8",
    "assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
    "primary_output": "majority",
    "unused_inputs": (),
    "subcircuits": (),
}

# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
    "v_b45380ba34de": {"kind": "input", "name": "A", "identifier": "input_A"},
    "v_d43875bbafcf": {"kind": "input", "name": "B", "identifier": "input_B"},
    "v_d39fa64298ba": {"kind": "input", "name": "C", "identifier": "input_C"},
    "f_44f6cbf56b13": {"kind": "output", "name": "majority", "identifier": "output_majority"},
}

INPUT_NAMES = ("A", "B", "C")
OUTPUT_NAMES = ("majority",)
ASSUMPTIONS = ()

_DECLARED_INPUTS = frozenset(INPUT_NAMES)
_SCOPE_INPUTS = ()
_ASSUMPTION_FORMULAS = {entry["name"]: entry["formula"] for entry in ASSUMPTIONS}


class InputError(ValueError):
    """Raised when inputs are missing, undeclared, or not exactly True/False.

    Every required input must satisfy type(value) is bool. Missing inputs, None, strings,
    integers (including 0 and 1), and keys that are not declared inputs are rejected before
    anything is evaluated. A missing input is never treated as False. The individual problems
    are listed in the problems attribute.
    """

    def __init__(self, problems):
        self.problems = tuple(problems)
        super().__init__("Invalid inputs: " + "; ".join(self.problems))


class OutOfScopeError(ValueError):
    """Raised when the inputs violate the model's assumptions (its declared scope).

    Outside its assumptions the rule has no decision: this error is never a False result.
    The names of the violated assumptions are in the violated attribute.
    """

    def __init__(self, violated):
        self.violated = tuple(violated)
        details = ", ".join(name + " (" + _ASSUMPTION_FORMULAS.get(name, "?") + ")" for name in self.violated)
        super().__init__(
            "Out of scope: the inputs violate "
            + ("assumption " if len(self.violated) == 1 else "assumptions ")
            + details
            + ". The rule has no decision outside its assumptions; this is not a False result."
        )


def _short_repr(value: object) -> str:
    text = repr(value)
    return text if len(text) <= 60 else text[:57] + "..."


def _checked_inputs(inputs: Mapping[str, bool], required: Tuple[str, ...], check_scope: bool) -> Dict[str, bool]:
    """Validate inputs strictly before anything is evaluated and return a plain copy."""
    if type(check_scope) is not bool:
        raise TypeError("check_scope must be True or False")
    if not isinstance(inputs, _abc.Mapping):
        raise InputError(["inputs must be a mapping from input names to True/False, got " + type(inputs).__name__])
    problems = []
    for key in inputs:
        if type(key) is not str or key not in _DECLARED_INPUTS:
            declared = ", ".join(repr(name) for name in INPUT_NAMES) or "none"
            problems.append("undeclared input " + _short_repr(key) + " (declared inputs: " + declared + ")")
    needed = tuple(required) + (_SCOPE_INPUTS if check_scope else ())
    for name in needed:
        if name not in inputs:
            problems.append("missing required input " + repr(name))
    for key, value in inputs.items():
        if type(key) is str and key in _DECLARED_INPUTS and type(value) is not bool:
            problems.append(
                "input " + repr(key) + " must be True or False (type bool), got "
                + type(value).__name__ + " " + _short_repr(value)
            )
    if problems:
        raise InputError(list(dict.fromkeys(problems)))
    return dict(inputs.items())


def _require_bool_args(function: str, pairs: Tuple[Tuple[str, object], ...]) -> None:
    problems = [
        "argument " + repr(name) + " of " + function + " must be True or False (type bool), got "
        + type(value).__name__ + " " + _short_repr(value)
        for name, value in pairs
        if type(value) is not bool
    ]
    if problems:
        raise InputError(problems)


def minterm_index(*values: bool) -> int:
    """Row index of an assignment over a declared variable order.

    The first value is the most significant bit, so all-False is row 0. Used for indexed
    minterm (sum) and maxterm (product) forms.
    """
    index = 0
    for value in values:
        if type(value) is not bool:
            raise InputError(["minterm_index() takes True/False values, got " + type(value).__name__ + " " + _short_repr(value)])
        index = index * 2 + (1 if value else 0)
    return index


def _scope_values(values: Dict[str, bool]) -> Tuple[Tuple[str, bool], ...]:
    """Value of every enabled, confirmed assumption for validated inputs, in declared order."""
    return ()


def _require_in_scope(values: Dict[str, bool]) -> None:
    violated = tuple(name for name, holds in _scope_values(values) if not holds)
    if violated:
        raise OutOfScopeError(violated)


def in_scope(inputs: Mapping[str, bool]) -> bool:
    """True when the inputs satisfy every assumption (the model's declared scope).

    Requires the inputs the assumptions read; raises InputError for invalid inputs.
    """
    values = _checked_inputs(inputs, (), True)
    return all(holds for _, holds in _scope_values(values))


# ---- Formula bodies (inputs are validated by the public functions below) ----


def _compute_output_majority(input_A: bool, input_B: bool, input_C: bool) -> bool:
    """Decision output 'majority': (A AND B) OR (A AND C) OR (B AND C)"""
    return (input_A and input_B) or (input_A and input_C) or (input_B and input_C)


# ---- Public API ----


def output_majority(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
    """Decision output 'majority' (stable ID f_44f6cbf56b13).

    'majority' := (A AND B) OR (A AND C) OR (B AND C)

    Required inputs: 'A', 'B', 'C'.
    Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
    when check_scope is True and the inputs violate an assumption.
    """
    values = _checked_inputs(inputs, ("A", "B", "C"), check_scope)
    if check_scope:
        _require_in_scope(values)
    output_majority_value = _compute_output_majority(values["A"], values["B"], values["C"])
    return output_majority_value


def evaluate(inputs: Mapping[str, bool], *, check_scope: bool = True) -> Dict[str, bool]:
    """Evaluate every output (declared order); each output is computed once.

    Required inputs: 'A', 'B', 'C'.
    Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
    when check_scope is True and the inputs violate an assumption.
    """
    values = _checked_inputs(inputs, ("A", "B", "C"), check_scope)
    if check_scope:
        _require_in_scope(values)
    output_majority_value = _compute_output_majority(values["A"], values["B"], values["C"])
    return {
        "majority": output_majority_value,
    }
PostgreSQL
-- Tangibool PostgreSQL export (tangibool.postgresql/1)
-- Target: PostgreSQL 16 or later with typed boolean inputs; tested with PostgreSQL 18.3 (PGlite 0.5.8).
-- Not a claim of compatibility with other SQL dialects.
-- Model: not stated
-- Source revision: reference
-- Interpretation status: not stated
-- Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
-- Logic digest: b7cf2f66ad74a4d23184d37a24dcffbc8278c9909728a0c4872e824905513fe8
-- Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
-- Mode: decision
--
-- Read-only: this file contains only WITH / VALUES / SELECT statements. No DDL, DML,
-- procedures, credentials, network access, or database changes. The expressions are pure
-- Boolean expressions, so their result does not depend on SQL evaluation order.
--
-- NON-NULL INPUT PRECONDITION: NULL is not a truth value of this rule. Every fragment below
-- is defined only when every input column it reads is a non-NULL boolean. The guard template
-- (section 2) marks any NULL input as 'invalid_input' and any row that violates the
-- assumptions as 'out_of_scope', and fills output columns only for 'ok' rows; a NULL in an
-- output column of another row is an envelope marker, not a third truth value. Never wrap
-- the fragments in COALESCE(..., FALSE) or otherwise choose a meaning for unknown inputs.
-- This evaluates the supplied model; it does not show that the model is a complete, legal,
-- or safe statement of a real-world rule.
--
-- Input columns (stable ID: name -> column):
--   v_b45380ba34de: A -> "in_A"
--   v_d43875bbafcf: B -> "in_B"
--   v_d39fa64298ba: C -> "in_C"
-- Output columns:
--   f_44f6cbf56b13: majority -> "out_majority"
-- Assumptions (scope K; exported separately and never folded into an output):
--   none; every complete input row is in scope.

-- ----------------------------------------------------------------------------------------------
-- 1. Expression fragments (parenthesised; non-NULL boolean inputs required)
-- ----------------------------------------------------------------------------------------------
--
-- [out_majority] output majority (stable ID f_44f6cbf56b13)
--   majority := (A AND B) OR (A AND C) OR (B AND C)
--   PRECONDITION: "in_A", "in_B", "in_C" are non-NULL booleans.
--   (("in_A" AND "in_B") OR ("in_A" AND "in_C") OR ("in_B" AND "in_C"))
--

-- ----------------------------------------------------------------------------------------------
-- 2. Guard template (read-only SELECT over an inputs list)
-- ----------------------------------------------------------------------------------------------
-- Replace the example rows between the markers with your own rows, or replace the inputs
-- CTE with a SELECT that provides the boolean columns listed in the header. status is
-- 'invalid_input' when any input is NULL, 'out_of_scope' when the assumptions are false, and
-- 'ok' otherwise; output columns are filled only for 'ok' rows (NULL elsewhere is an
-- envelope marker). Assumption columns are shown for rows with complete inputs.
WITH inputs ("row_id", "in_A", "in_B", "in_C") AS (
  -- tangibool:example-rows:begin
  VALUES
    (1, FALSE, FALSE, FALSE),
    (2, FALSE, FALSE, TRUE),
    (3, FALSE, TRUE, FALSE),
    (4, FALSE, TRUE, TRUE),
    (5, TRUE, FALSE, FALSE),
    (6, TRUE, FALSE, TRUE),
    (7, TRUE, TRUE, FALSE),
    (8, TRUE, TRUE, TRUE),
    (9, NULL::boolean, FALSE, FALSE)
  -- tangibool:example-rows:end
),
checked AS (
  SELECT
    inputs.*,
    CASE
      WHEN "in_A" IS NULL OR "in_B" IS NULL OR "in_C" IS NULL THEN 'invalid_input'
      ELSE 'ok'
    END AS "status"
  FROM inputs
)
SELECT
  "row_id",
  "in_A",
  "in_B",
  "in_C",
  "status",
  CASE WHEN "status" = 'ok' THEN (("in_A" AND "in_B") OR ("in_A" AND "in_C") OR ("in_B" AND "in_C")) END AS "out_majority"
FROM checked
ORDER BY "row_id";
SMT-LIB
; Tangibool SMT-LIB export (tangibool.smtlib/1)
; Profile: SMT-LIB 2.6 Boolean subset (QF_UF): Bool constants, define-fun, let, push/pop, assert, check-sat, echo.
; Tested with Z3 5.1.0 (z3-solver 5.2.0, WebAssembly build).
; Model: not stated
; Source revision: reference
; Interpretation status: not stated
; Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
; Logic digest: b7cf2f66ad74a4d23184d37a24dcffbc8278c9909728a0c4872e824905513fe8
; Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
; Mode: decision
;
; Reading results: each query block states what sat and unsat mean for its question. A sat
; answer comes with a witness or counterexample (ask for it with (get-model) after sat only);
; unsat settles that scoped query and is solver-checked, not an independently checked proof
; certificate; unknown settles nothing. Blocks are scoped with push/pop, so assertions never
; accumulate across questions. This checks the supplied model, not whether it is a complete,
; legal, or safe statement of a real-world rule.
;
; Symbols (stable ID: name -> symbol):
;   v_b45380ba34de: A -> in_A
;   v_d43875bbafcf: B -> in_B
;   v_d39fa64298ba: C -> in_C
;   f_44f6cbf56b13: majority -> out_majority

(set-option :produce-models true)
(set-info :smt-lib-version 2.6)
(set-logic QF_UF)

; ---- Inputs (declared order)
(declare-const in_A Bool) ; A
(declare-const in_B Bool) ; B
(declare-const in_C Bool) ; C

; ---- Outputs (dependencies first)
; output majority (stable ID f_44f6cbf56b13): (A AND B) OR (A AND C) OR (B AND C)
(define-fun out_majority () Bool (or (and in_A in_B) (and in_A in_C) (and in_B in_C)))

; ---- Assumptions (the scope K; never folded into an output)
; K: conjunction of the enabled, confirmed assumptions (true when there are none)
(define-fun K () Bool true)

; ---- Definitions only: nothing is asserted and no check is run.
; Add (assert ...) and (check-sat) inside (push 1) ... (pop 1) to ask a question.
LaTeX
majority = \left(A \cdot B\right) + \left(A \cdot C\right) + \left(B \cdot C\right)

The common slip

Expecting the simplifier to shorten the formula because the three terms overlap. They do overlap, on the row where all three vote yes, but removing any term loses one of the two-yes rows. The table shows all four true rows survive only with all three terms present.

Questions

Why does the CNF have the same size as the DNF here?

Majority is self-dual: swapping AND with OR gives back the same function. So (A OR B) AND (A OR C) AND (B OR C) has three clauses of two literals, exactly mirroring the three two-literal terms of the DNF, and the truth table confirms both forms on every row.

How does the 9-gate NAND form relate to the formula?

Each A AND B pair becomes a NAND, and the OR of the three pairs becomes a NAND of their outputs, which needs the usual tied-input inverters in the mechanical construction. The converter reports 9 gates and replays the result against the table before showing it.

Related

Guides

Operators and gates

Tools

See also

Worked examples

Generated from the Tangibool engine, semantics tangibool-semantics/1, engine 1.0.0, build . Every table, map, simplification and verdict on these pages was computed by the Tangibool engine when the site was built, and every claim was re-checked by the same exact checker the app uses. Limits are stated where they apply. © 2026 Foreranger LTD.