Paired left and right curly brackets are used in mathematics and chemistry to enclose elements already enclosed by parentheses and square brackets:
x + {1 + [3 + 2 × (4 + 5)]}
In maths, a left curly bracket covering several lines is used for grouping:
\(f(x)=\begin{cases}x&\text{if }x\ge0\\-x&\text{if }x<0\end{cases}\)