Combining Reverse Solidus Overlay

It marks a negative value or negates a relation, depending on context.

On a counting board, people arranged small rods to calculate; rods of a different color marked negative numbers. In written Chinese counting-rod numerals, a diagonal reverse slash over the rightmost digit marks a negative value. This is a specialized numeral convention, not a general-purpose “no.”

Slashes also began as marks added to digits during calculations and later moved into printed algebra and calculus. Different strike directions appear in historical manuscripts by the mathematician Leibniz and can point to different calculation traditions, so a crossed-out digit needs context.

How do I put ⃥ over another character?

Put it after the base character.

Write the base followed by ⃥, as in =⃥. Documented sequences also include ≡⃥ and ⫽⃥; in each, the base tells you which relation is being negated.

The input order is base first, then ⃥. Typing the mark by itself can leave a blank or floating mark because it has no base to modify.

A missing slash points to font support

A font may contain ⃥ without the attachment support needed to position it over a base. The exact stroke placement depends on the font and renderer; plain text does not prescribe it.