• Syntax: \left|...\right|
• Ex.: $$$\left|b-a\right|$$$ gives $$\left|b-a\right|$$
• Ex.: $$${\rm~det}\left|\begin{array}{cc}a&b\\c&d \end{array}\right|$$$ gives  

$${\rm~det}\left|\begin{array}{cc}a&b\\c&d \end{array}\right|$$ 

 
("\rm~something" renders "something" in roman style)