Search:

A progression is a series of grits used to slowly achieve a final polish. An example of a progression could look something like 1k, 4k, 8k, 12k.

The way Slartibartfast used the word means that...

Exactly. Or, with the norton 4k/8k you can actually do pyramids as described in the wiki, where you flip back and forth between the two sides.

Are you asking why a single stone would contain two grits? If so, Slartibartfast nailed that one. You basically get half of a full thickness 4k and half of a full thickness 8k for the price of one...