Slide rule bezel watches

A slide rule bezel is a type of bezel used to perform various calculations like unit conversions, fuel consumption, speed and distance calculations. The bezel usually features two separate markings, one of the rings representing time while another ring representing various units like miles, kilometers or nautical miles.

You do the calculations by aligning the two separate bezel parts to one another. For example to convert miles to kilometers you need to know the factor of conversion, in this case 1 mile is roughly 1.6 km. Knowing this you align the outer scale representing miles to the inner scale conversion factor, 1.6. Now you can look at the desired conversion number on the outer scale and you will have the converted number on the inner scale.

This was a feature primarily developed for pilots and navigators in a time when we didn't have digital equipment to do all these calculations for us. The tool still works if you learn how to use it but its no longer a necessity.