"Up tack" is the Unicode name for a symbol (⊥, \bot in LaTeX, U+22A5 in Unicode[1]) that is also called "bottom",[2] "falsum",[3] "absurdum",[4] or "the absurdity symbol",[5][6] depending on context. It is used to represent:
The truth value'false', or a logical constant denoting a proposition in logic that is always false. (The names "falsum", "absurdum" and "absurdity symbol" come from this context.)
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).[7][8] 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:
Historically, in character sets before Unicode 4.1 (March 2005), such as Unicode 4.0[9] and JIS X 0213, the perpendicular symbol was encoded with the same code point as the up tack, specifically U+22A5 in Unicode 4.0.[10] This overlap is reflected in the fact that both HTML entities ⊥ and ⊥ refer to the same code point U+22A5, as shown in the HTML entity list. In March 2005, Unicode 4.1 introduced the distinct symbol "⟂" (U+27C2 "PERPENDICULAR") with a reference back to ⊥ (U+22A5 "UP TACK") and a note that "typeset with additional spacing."[11]
