Two double-headed arrows, one asserting equivalence and one asserting that neither drawing is real.
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.
Both are double-headed arrows between two expressions, in unrelated fields.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | If-and-only-if sign | Resonance arrow |
|---|---|---|
| Symmetry | horizontal | point |
| If-and-only-if sign | Resonance arrow | |
|---|---|---|
| What it asserts | Each statement implies the other | Neither structure is real on its own |
| Shaft | Doubled | Single |
| What sits either side | Statements | Structural formulae |
| Are both sides real? | Yes, and equivalent | No — that is the point |
| Relationship (from Symbolwise's records) | Commonly confused with Resonance arrow — Both are double-headed arrows between two expressions, in unrelated fields. | Commonly confused with If-and-only-if sign — Both are double-headed arrows between two expressions, in unrelated fields. |
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.
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Count the shafts and look at the heads. Equilibrium is two separate half-headed arrows pointing opposite ways: two real species interconverting, both present. Resonance is one shaft with a full head at each end: one species that neither drawing captures. Reading resonance as equilibrium claims two substances exist where there is only 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.
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.
The becquerel counts nuclear decays — one per second — and says nothing about harm. The sievert weights absorbed energy by the kind of radiation delivering it, and so estimates biological effect. A becquerel of one isotope and a becquerel of another can carry very different risk, which is exactly why exposure limits and medical doses are stated in sieverts and never in becquerels.