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A double-shafted arrow with heads at both ends, denoting logical equivalence.
Also known as: Iff, Double-headed arrow
⇔ denotes equivalence: each statement implies the other. The second arrowhead is the whole difference from one-directional implication, and it is the difference between 'if' and 'if and only if'.
Is encoded as U+21D4 LEFT RIGHT DOUBLE ARROW, in the Arrows block.
Contexts: Mathematics
States that each of the two statements implies the other, so they hold in exactly the same circumstances.
Contexts: Mathematics
Is abbreviated 'iff' in written mathematics, a deliberate doubling of the f to distinguish equivalence from the one-directional 'if'.
Contexts: Mathematics
A horizontal arrow whose shaft is drawn as two parallel lines, with an open arrowhead at each end.
Both are double-headed arrows between two expressions, in unrelated fields.
Both directions against one; the second head is the difference.