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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.
Look for the horizontal line. Fire is a plain upward triangle; air is the same triangle with a bar drawn across its upper part. Dropping the bar is the commonest error in reproductions of the element set, and it turns air into fire.
The same rule as fire and air, inverted. Water is a plain downward triangle; earth is the same triangle with a bar across its lower part. The four element signs are one designed set: two point up, two point down, and within each pair the bar is the only difference.