Linux Man Page Numbers

Why do man pages have numbers? The numbers, and sometimes letters (often shown in parentheses), correspond to man page "sections".

For example ls(1) is the ls command in section 1, also known as "General Commands".

Man page section number reference
# Section Name Description
1 General Commands User commands and tools, for example, file manipulation tools, shells, compilers, web browsers, file and image viewers and editors, and so on.
2 System Calls A system call is an entry point into the Linux kernel. Usually, system calls are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required in order to invoke the system call.
3 Library Functions All library functions excluding the library functions (system call wrappers) described in section 2, which implement system calls.
4 Special Files Devices and drivers.
5 File Formats File formats and protocols, and the corresponding C structures, if any.
6 Games Games and screensavers.
7 Miscellanea Conventions and protocols, character set standards, the standard file system layout, and miscellaneous other things.
8 System Administration Commands which either can be or are used only by the superuser, like system-administration commands, daemons, and hardware-related commands.
0p POSIX Header Files POSIX Header Files
1p POSIX Commands POSIX Commands
3p POSIX Functions POSIX Functions
n Tcl/Tk Keywords Tcl/Tk Keywords