Staircase light with three switches: A XOR B XOR C
A staircase light wired to three switches, A XOR B XOR C: flipping any one switch toggles the light. The eight-row table, the four-term normal forms and a map with four isolated ones show it is the parity function.
Three switches control one lamp, and the rule is that any single flip changes the state. The formula that does this is the chained XOR: the light is on when an odd number of switches are up. The engine's reading says light is true exactly when an odd number of A, B and C are true, one or three of them. On the eight-row table the output is true on rows 1, 2, 4 and 7, Σm(1, 2, 4, 7), and each of those rows differs from each false row in exactly one switch, which is what makes every flip a toggle.
The checks: not a tautology, counterexample A = FALSE, B = FALSE, C = FALSE; satisfiable, witness A = FALSE, B = FALSE, C = TRUE. The simplifier found no law that shortens the two-operator expression. The normal-form converter, by XOR elimination twice and distribution twice, produces a four-term DNF, (A AND B AND C) OR (A AND NOT B AND NOT C) OR (NOT A AND B AND NOT C) OR (NOT A AND NOT B AND C), and a four-clause CNF, twelve literals each. The Karnaugh map draws four single-cell groups, four prime implicants. Gate counts: 20 NAND, 41 NOR.
Electricians build this with two three-way switches and a four-way switch in the middle, and the wiring is a physical XOR chain. The same function runs any “toggle from anywhere” feature in software: a setting that several controls can flip, where each control inverts the current state rather than setting it. The thing to watch is the absolute position: with XOR no switch position means on or off by itself, only the parity does, which surprises users who expect “up means on”. The parity page is this same function under its hardware name.
light := A XOR B XOR C · output
Exact checks
Truth table (8 rows)
| # | A | B | C | light |
|---|---|---|---|---|
| 0 | FALSE | FALSE | FALSE | FALSE |
| 1 | FALSE | FALSE | TRUE | TRUE |
| 2 | FALSE | TRUE | FALSE | TRUE |
| 3 | FALSE | TRUE | TRUE | FALSE |
| 4 | TRUE | FALSE | FALSE | TRUE |
| 5 | TRUE | FALSE | TRUE | FALSE |
| 6 | TRUE | TRUE | FALSE | FALSE |
| 7 | TRUE | TRUE | TRUE | TRUE |
Plain-language reading
Beginnerlight is true exactly when an odd number of A, B, and C are true (1 or 3 of them).
In every other case, light is false.
About the operators:
- A chain of XORs is true when an odd number of its parts are true. With three parts, all three being true also counts: it does not mean “exactly one”.
Intermediatelight is true exactly when an odd number of A, B, and C are true (1 or 3 of them).
In every other case, light is false.
Advancedlight is true exactly when an odd number of A, B, and C are true (1 or 3 of them) [parity: n-ary exclusive disjunction ⊕].
In every other case, light is false.
Formal notation: A ⊕ B ⊕ C
Logic diagram
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 XOR B XOR C
Karnaugh map
- cells m1
NOT A AND NOT B AND C - cells m2
NOT A AND B AND NOT C - cells m4
A AND NOT B AND NOT C - cells m7
A AND B AND C
Minterms and maxterms
light(A, B, C) = Σm(1, 2, 4, 7)light(A, B, C) = ΠM(0, 3, 5, 6)Variable order: A, B, C · 8 rows
Normal forms
Conjunctive normal form (CNF)
(A OR B OR C) AND (A OR NOT B OR NOT C) AND (NOT A OR B OR NOT C) AND (NOT A OR NOT B OR C)4 clauses, 12 literals · complete · XOR elimination ×2, Distribution ×2
Disjunctive normal form (DNF)
(A AND B AND C) OR (A AND NOT B AND NOT C) OR (NOT A AND B AND NOT C) OR (NOT A AND NOT B AND C)4 terms, 12 literals · complete · XOR elimination ×2, Distribution ×2
NAND-only and NOR-only
NAND-only form
NAND(NAND(NAND(NAND(A, NAND(B, B)), NAND(NAND(A, A), B)), NAND(C, C)), NAND(NAND(NAND(NAND(A, NAND(B, B)), NAND(NAND(A, A), B)), NAND(NAND(A, NAND(B, B)), NAND(NAND(A, A), B))), C))20 gates · verified equivalent on every row
NOR-only form
NOR(NOR(NOR(NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))), C), NOR(NOR(NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))), NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B)))), NOR(C, C))), NOR(NOR(NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))), C), NOR(NOR(NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))), NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B)))), NOR(C, C))))41 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: 53ee60e932ba3e779f694db0406de7228505c1c8e2c1466fe3788480ca81e0a4
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:
'light' -> output_light(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})
# -> {'light': False}
model.output_light({'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_light",
]
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": "53ee60e932ba3e779f694db0406de7228505c1c8e2c1466fe3788480ca81e0a4",
"assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
"primary_output": "light",
"unused_inputs": (),
"subcircuits": (),
}
# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
"v_98df8f0d0a0a": {"kind": "input", "name": "A", "identifier": "input_A"},
"v_3f0bce171fa5": {"kind": "input", "name": "B", "identifier": "input_B"},
"v_886ba5b964aa": {"kind": "input", "name": "C", "identifier": "input_C"},
"f_f83bf3dae53d": {"kind": "output", "name": "light", "identifier": "output_light"},
}
INPUT_NAMES = ("A", "B", "C")
OUTPUT_NAMES = ("light",)
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_light(input_A: bool, input_B: bool, input_C: bool) -> bool:
"""Decision output 'light': A XOR B XOR C"""
return (input_A != input_B) != input_C
# ---- Public API ----
def output_light(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'light' (stable ID f_f83bf3dae53d).
'light' := A XOR B XOR 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_light_value = _compute_output_light(values["A"], values["B"], values["C"])
return output_light_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_light_value = _compute_output_light(values["A"], values["B"], values["C"])
return {
"light": output_light_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: 53ee60e932ba3e779f694db0406de7228505c1c8e2c1466fe3788480ca81e0a4
-- 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_98df8f0d0a0a: A -> "in_A"
-- v_3f0bce171fa5: B -> "in_B"
-- v_886ba5b964aa: C -> "in_C"
-- Output columns:
-- f_f83bf3dae53d: light -> "out_light"
-- 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_light] output light (stable ID f_f83bf3dae53d)
-- light := A XOR B XOR C
-- PRECONDITION: "in_A", "in_B", "in_C" are non-NULL booleans.
-- (("in_A" <> "in_B") <> "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" <> "in_B") <> "in_C") END AS "out_light"
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: 53ee60e932ba3e779f694db0406de7228505c1c8e2c1466fe3788480ca81e0a4 ; 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_98df8f0d0a0a: A -> in_A ; v_3f0bce171fa5: B -> in_B ; v_886ba5b964aa: C -> in_C ; f_f83bf3dae53d: light -> out_light (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 light (stable ID f_f83bf3dae53d): A XOR B XOR C (define-fun out_light () Bool (xor (xor in_A 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
light = A \oplus B \oplus C
The common slip
Wiring the switches as an OR, so the light is on when any switch is up. Then flipping a second switch up changes nothing, and the room cannot be darkened from the top of the stairs. The table shows the difference on A = TRUE, B = TRUE, C = FALSE: XOR gives off, OR gives on.
Questions
Does it matter which switch is which?
No. XOR is commutative and associative, so the three switches are interchangeable and the grouping of the chain does not change the column. The table is symmetric under any renaming of A, B and C, which is also why the four true rows are the ones with one or three switches up.
Related
Operators and gates
Tools
- Truth table generator: any Boolean expression, up to 12 variables, with intermediate columns
- Boolean algebra calculator and simplifier with steps (every law named and verified)
- Karnaugh map solver: 2 to 4 variables, groups, SOP and POS, don’t-cares from assumptions
See also
- Odd parity of three inputs: A XOR B XOR C
- Full adder sum: A XOR B XOR Cin
- XOR gate: truth table for 2, 3 and 4 inputs (odd parity, not “exactly one”)
- Lesson 2: XOR: exclusive OR and parity (beginner, 10 min)
- XOR of three inputs is odd parity, not “exactly one”
- Associative law: (A AND B) AND C = A AND (B AND C)
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.