I have 2 domains those belong to me:
UceWorld.com was the domain of my blog, when I hosted it with OGSite.
IamThan.com was the domain of my blog when I hosted it with Dreamhost.
Now, I've paid $10 for each domain to be mapped to thanr.wordpress.com, the mapping and the transfer process have been completed.
So, the primary domain will be IamThan.com, though you can access this blog with the same mapping but it will be directed to this domain.