• Syntax: \left{...\right.  (note the dot at the end!)
• Ex.: $$f(x)=\left{{x^2, \rm~if x>-1\atop~0, \rm~else}\right.$$ gives

$f(x)=\left{{x^2, \rm~if x>-1\atop~0, \rm~else}\right.$

(\rm~something switches to roman style)