he expression
located in the denominator of the ( Swap rate) is a
linear combination of prices of traded instruments. It is always positive.
Accoring to ( Suitable numeraire)
may be taken as a numeraire. The probability measure associated with the
numeraire
is called the "swap measure". The numerator of the ( Swap
rate) is also a price of a traded instrument. Therefore, the quantity
is a martingale under the swap measure.
One may justify the use of Black-Scholes formula for pricing of a swaption
using the notion of swap measure. Indeed, the price of the swaption is given
by   
We arrived to the Black-Scholes situation if
is assumed to be log-normal.
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