# Use of the dot in math-mode

I was trying to typesset piecewise functions earlier and I had a problem typesseting only the left brace and not the right as it couldn't accept only \left\{. After that I tried adding \right but there were still errors. Until I found out this piece of code in wikipedia showing a dot after \right. What is the use of this dot?

$f(x) = \left\{ \begin{array}{lr} x^2 & : x < 0\\ x^3 & : x \ge 0 \end{array} \right.$

• Simply expressed: The dot fakes a closing 'bracket' when you opened a left one. Normally, LaTeX expects a right 'bracket', omitting it will lead to errors. The dot's job is to prevent this – user31729 Oct 13 '14 at 14:28
• . has delimiter code 0 which means that it is a legal delimiter that can go after \left or \right it causes the horizontal space \nulldelimiterspace to be added and allows \left \right pairs to match. – David Carlisle Oct 13 '14 at 14:30

## 1 Answer

A \left\{ should always be matched with a \right\} in the same line. This will put the opening and closing braces both. In case if you want only the opening brace (\left\{) then you need to match it with an invisible \right. The . is for invisible pair. Same thing is true for \left. if you want only the closing pair.