Up Tack

In logic, ⊥ is a statement that can never be true. Called falsum or bottom, it marks absurdity: a proof that derives ⊥ has hit a contradiction, which is how a proof by contradiction ends. Some logics even define “not φ” as φ → ⊥, “φ leads to absurdity.”

Outside logic, bottom means the lowest element of an ordering. In a lattice (a structure where any two elements have a greatest lower bound), ⊥ sits below everything. In programming-language theory, the bottom type has no values at all, so a function returning it never returns normally.

Is ⊥ the perpendicular symbol?

Traditionally yes; in current Unicode, use ⟂. Older character sets and the HTML entity ⊥ reuse ⊥ for perpendicular lines, so pasted geometry like AB ⊥ CD is still common. Unicode proposed a dedicated perpendicular relation, ⟂, in 2003 and encoded it in 2005, spaced as a relation between two objects.