A bidirectional horizontal arrow symbol representing logical equivalence, biconditional statements, or two-way relationships between expressions.
Examples
Logical equivalence between statements
P \harr Q \text{ means 'P if and only if Q'}
Mathematical equivalence
x^2 = 4 \harr x = \pm 2
Set equivalence
A = B \harr (x \in A \harr x \in B)