At current job I deal with some billing software. There's a table of payments containing such rows as PAYMENT_ID (primary key), date of payment, amount of money, currency code etc. Any payment may come from a limited number of sources (cash, bank check, ATM etc). Each source of payment define its unique (and rare used) set of payment attributes (transaction number, cash register identifier etc).

So there are several tables for supporting these attributes (say CASH_PAYMENT_SPEC, BANK_PAYMENT_SPEC, ATM_PAYMENT_SPEC). Each table has a primary key containing one column (PAYMENT_ID) and a foreign key constraint that references the dbo.PAYMENT.PAYMENT_ID column (i.e., those foreign keys just implement 1-1 relationships).