Mutations

The 1 1/3' and 1 3/5' stops are easily confused. Here are two simple ways to distinguish between them:

• Mathematics (division). 1 1/3' and 2 2/3' are closely related mathematically. A 1 1/3' pipe (not the 1 3/5') is half the length of a 2 2/3' pipe. Remember that halving of pipe length raises pitch by an octave. Hence, 1 1/3', like 2 2/3', is a fifth-sounding rank, whereas 1 3/5' is a third-sounding rank.
• Opposites. The mutation that places 5 in the denominator (1 3/5') is not the fifth, but the third-sounding rank. The one that places 3 in the denominator (1 1/3') is not the third, but the fifth-sounding rank.