@User: "scalegrid" actually changes the database dimensions. "grid" just creates a visual reference grid, and you can snap to it (or not) with the "snap" command. You can see the effect of "scalegrid" by starting magic and doing "box size 1um 1um ; box ; scalegrid 2 1 ; box". In the dimensions returned by the "box" command (no arguments), you'll see that the box is still 1um x 1um, but the internal units are different by a factor of 2.

Ok, thanks, I understand now