\underbar{X}The command \underbar{X} is used to place a horizontal line (or bar) directly underneath the character or group of characters X. This is often used in mathematical notation to denote vectors, matrices, or to emphasize certain variables or constants.
A single character 'a' with an underbar, often used to denote a vector.
\underbar{a}A group of characters 'AB' with an underbar, which could represent a matrix or a vector.
\underbar{AB}A Greek letter theta with an underbar, used to emphasize or denote a specific parameter or variable.
\underbar{\theta}