Convention/standard on using curly braces around options

The man page for tar in Arch Linux **SYNOPSIS** section starts with: tar {A|c|d|r|t|u|x}[GnSkUWOmpsMBiajJzZhPlRvwo] [ARG...] I perfectly understand that in this situation it means that exactly one of the options in the curly braces must be given because it says so further down in the man page. Is there some kind of standard or convention on using {} around a list of mutually exclusive options? All I can find is a User's group tutorial on man from 1998 where it says: > Some options will have a limited list of choices. A list of choices > will be comma separated and put between braces. > > {choice1,choice2} > {yes,no} Seems like every convention (like this one from the Open Group ) or man page describes only square brackets ([]) or vertical lines (|).