a question about Latex

I used subsubsection in latex doesn't work.
For example:

\ chapter{a}
\ section{b}
\ subsection{c}
\ subsubsection{d}

it should display like this:
1. a
1.1 b
1.1.1 c
1.1.1.1 d

but it displays as following:
1. a
1.1 b
1.1.1 c
d

anybody has an idea? Thank you