severn or yarrow ?

[Hunt,Russ]

When I started downloading the ISO's for fedora on Tuesday the version
listed at the fedora site was severn-i386-disc1.iso etc.
For various reasons I've only downloaded so far discs 1 + 2 and this morning
notice the ISO's listed as yarrow 1-3 rather than severn 1-3
I see there are various mirror sites from where I cane still get
severn-i386-disc3.iso but what is the difference between severn and yarrow ?

Regards

Russ