Yum $releasever $basearch

[MJang]

Folks, 

Just wondering where the $releasever and $basearch variables are set
(these variables are found in the files in the /etc/yum.repos.d
directory. 

I understand what they are, that $releasever is the release version
(i.e. 3 or 4 for Fedora Core 3 or 4), and $basearch is the architecture
such as i386 or x86_64, but just can't figure out where these variables
are set.

Thanks,
Mike