  The main result of this paper that a martingale evolution can be chosen for
Libor such that all the Libor interest rates have a common market measure; the
drift is fixed such that each Libor has the martingale property. Libor is
described using a field theory model, and a common measure is seen to be emerge
naturally for such models. To elaborate how the martingale for the Libor
belongs to the general class of numeraire for the forward interest rates, two
other numeraire's are considered, namely the money market measure that makes
the evolution of the zero coupon bonds a martingale, and the forward measure
for which the forward bond price is a martingale. The price of an interest rate
cap is computed for all three numeraires, and is shown to be numeraire
invariant. Put-call parity is discussed in some detail and shown to emerge due
to some non-trivial properties of the numeraires. Some properties of swaps, and
their relation to caps and floors, are briefly discussed.
