Combining Left Right Arrow Above

It modifies the preceding symbol; in physics, it can mark a tensor. A 1987 revision of the International Union of Pure and Applied Physics style guide recommends double-arrow notation for a second-rank tensor when a distinctive typeface is unavailable. A second-rank tensor is a quantity described by two indices, the labels used to identify its components. The guide was reprinted in 2010; the equation identifies which tensor the writer means.

Geometry uses a related, stretchable overarrow: \overleftrightarrow{AB} marks the full line through points A and B, extending without end in either direction. It spans both labels, so it is not this single combining character.

The broader vector-notation family developed through Hamilton, Grassmann, and Gibbs in the nineteenth century; Wilson's 1901 textbook helped spread modern forms.

Why does a dotted circle appear around it?

It stands in for the missing base character. A dotted circle in a character chart or isolated display illustrates where this mark attaches; the circle is not part of the mark. In text, put the mark after its base character. Fonts and renderers determine how it is placed and displayed.