Single brace

Summary

Used for emphasizing that several equations have to be considered as simultaneous equations; for example, $\textstyle \begin{cases}2x+y=1\\3x-y=1\end{cases}$.

Piecewise definition; for example, $\textstyle |x|=\begin{cases}x&\text{if }x\ge 0\\-x&\text{if }x< 0\end{cases}$.

Used for grouped annotation of elements in a formula; for example, $\textstyle \underbrace{ (a,b,\ldots,z) }_{26}$, $\textstyle \overbrace{ 1+2+\cdots+100 }^{=5050}$, $\textstyle \left.\begin{bmatrix}A\\B\end{bmatrix}\right\} m+n\text{ rows}$