• 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)