Partial Fullfilling & invoicing of Sales order

Is there a weay to generate partial invoices on a Sales Order - and keep track of what you have invoiced ?

Say a customer gives a po for 2 widget A, and 5 widget B. You deliver 1 widget A and 1 Widget B. So you want to invoice these. And you want to keep track of what you have fulfilled from the Sales Order to know you still have to deliver and invoice the rest.

I do not see how this is done. If it is not done, can it be added ? Otherwise Convert Sales Order to Invoice, is pretty mute.