Examples of common derivative contracts include interest rate swaps, currency forwards, commodity futures, and equity options. The UNDERLYING for an interest rate swap is:

An interest rate swap exchanges cash flows based on movements in interest rates — interest rates are the underlying variable. The notional principal is just the amount on which interest is computed, the credit rating influences risk pricing but is not the swap's underlying, and the FX rate is the underlying of a currency swap (a separate derivative type).