The 4, 5.6, 8, 11, 16, 22, 32, 45 ... sequence results in exposures that are each 1/2 stop different from the adjacent ones.

The images that result differ in density in what appears to be even steps - due to the logarithmic response of the paper.

So the appearance of the images that result make it much easier to evaluate appropriate exposure including, where necessary, interpolating between two examples.

It works really well, and the sequence is easy to remember .