U+27DF Up Tack with Circle Above

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The up tack or falsum (⊥, ot in LaTeX, U+22A5 in Unicode) is a constant symbol used to represent:

• The truth value 'false', or a logical constant denoting a proposition in logic that is always false (often called "falsum" or "absurdum").
• The bottom element in wheel theory and lattice theory, which also represents absurdum when used for logical semantics
• The bottom type in type theory, which is the bottom element in the subtype relation. This may coincide with the empty type, which represents absurdum under the Curry–Howard correspondence
• The "undefined value" in quantum physics interpretations that reject counterfactual definiteness, as in (r₀,⊥)

as well as

• Mixed radix decoding in the APL programming language

The glyph of the up tack appears as an upside-down tee symbol, and as such is sometimes called eet (the word "tee" in reverse). Tee plays a complementary or dual role in many of these theories.

The similar-looking perpendicular symbol (⟂, perp in LaTeX, U+27C2 in Unicode) is a binary relation symbol used to represent:

• Perpendicularity of lines in geometry
• Orthogonality in linear algebra
• Independence of random variables in probability theory
• Coprimality in number theory

The double tack up symbol (⫫, U+2AEB in Unicode) is a binary relation symbol used to represent:

• Conditional independence of random variables in probability theory