How do I reference an AP Check to the Voucher in a Sql query?

I can't believe that I can figure this out. 

I need a query of AP Vouchers and their related checks.  I am expecting to see a reference in either the APDoc or APTran table of the Check record that references the Voucher RefNbr.  ApplyRefNbr seemed like a likely candidate but it always seems to be the same as RefNbr.  Clearly some kind of reference exists because the check screen displays the voucher information and I can drill down to the voucher.

What am I missing?