Sometimes you need to know the reciprocal of something. That means you want to know what 1 divided by a number is. The C1 scale makes it easy. Just find the number on the C scale with the hairline and read its reciprocal directly above on the C1 scale. Or, you could use the D and D1 scales. If you need to know the reciprocal of another number, just move the hairline and read the answer.
In the graphic the pale blue letters mark the C and C1 scales. The hairline is over 2 on the C scale. The reciprocal of 2 is 0.5, as you can see on the C1 scale under the hairline. Without moving the slider, you can also see by the blue arrows that 0.4 is the reciprocal of 2.5, and by the red arrows that 0.333 is the reciprocal of 3.