una corda/tre corda

[David Love]

Since the una corda shifts and hits two strings, why is it called the una
corda rather than the due corda.  I assumed that this might have been
because in the early days there were only two string unisons, so that when
the una corda was depressed it did, in fact, strike only one string.  If
that were true, then why, when the una corda is released, is the
instruction given as "tre corda".  

Clear this one up for me please.  


David Love
davidlovepianos@earthlink.net