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The **up tack** or **falsum** (**⊥**, `\bot`

in LaTeX, U+22A5 in Unicode^{[1]}) 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 also represents absurdum under the Curry–Howard correspondence

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).^{[citation needed]} 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^{[2]}) is a constant symbol used to represent: