\llcorner \lrcornerThe symbols \llcorner and \lrcorner are used in LaTeX to represent the lower left and lower right corners, respectively. These are often used as delimiters in mathematical expressions to denote specific types of brackets or to highlight certain parts of a diagram or matrix.
Matrix with corner delimiters
A = \llcorner 1 & 0 \\ 0 & 1 \lrcornerOrdered pair with corner delimiters
\llcorner x, y \lrcornerExpression enclosed with corner delimiters
\llcorner a + b \lrcorner