How to build for i686 on x86_64 platform

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http://forums.fedoraforum.org – I'm trying to build from a source rpm packages for both i686 and x86_64. The build platform is F18 x86_64 and the build for the x86_64 arch is successful. I have set up the build environment under the username 'makerpm' and followed the package building documentation on the Fedora wiki. I think the problem is that i686 libraries are needed. I just didn't see how to set it up. (HowTos)