One arrowhead or two, and the difference is between a one-way claim and a two-way one.
Count the arrowheads. A single head means implication: if the left statement holds then so does the right, with no claim in the other direction. Two heads mean equivalence: each statement implies the other, so they hold in exactly the same circumstances. Reading a one-way implication as though it were equivalence is the commonest error made with either symbol.
Both directions against one; the second head is the difference.
| If-and-only-if sign | Implies sign | |
|---|---|---|
| What it claims | Each statement implies the other | The left implies the right, and only that |
| Arrowheads | Two, one at each end | One, at the right |
| Read aloud as | If and only if — written 'iff' | Implies, or 'if … then' |
| Code point | U+21D4 | U+21D2 |
| Relationship (from Symbolwise's records) | Commonly confused with Implies sign — Both directions against one; the second head is the difference. | Commonly confused with If-and-only-if sign — One direction against both; the difference is a second arrowhead. |
Unicode Code Chart: Arrows (U+2190–U+21FF)
The Unicode Consortium · retrieved 2026-07-31
Authoritative for the character's code point, name and block only. Unicode assigns identity; it does not define what a symbol means in use.
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for what the notation means in ordinary mathematical practice and for attribution of its introduction, not for a claim that any body defines it.
Glossary of mathematical symbols
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for what the notation means in ordinary mathematical practice and for attribution of its introduction, not for a claim that any body defines it.
The logical arrow has a doubled shaft and joins two statements that hold in exactly the same circumstances. The chemical one has a single shaft, joins two structural formulae, and means something stranger: neither structure exists, and the real molecule is a single thing no drawing captures.
In logic it asserts that the right-hand statement follows from the left-hand one. In JavaScript the same arrow separates a function's parameters from its body. Both are drawn as two horizontal strokes followed by an angle bracket, and the notations are otherwise unconnected.
Look at the shaft and at what surrounds it. Logical implication is drawn with a doubled shaft and sits between statements; a reaction arrow has a single shaft and sits between chemical formulae, usually with conditions written above and below it. Neither is an equals sign, and both assert direction rather than equality.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.