From Wikinline

Mount Spacing

The mount spacing is the separation between the front and read mount points. Both the boot and the frame to be mounted together must be compatible with the same spacing.

The most common spacing is 165mm, while UFS is 167mm and many speed skates use 195mm.

165mm Spacing

The 165mm spacing is the most common. The rear mount is 10mm higher than the front mount.

UFS Spacing

See UFS frames for details. The Universal Frame System uses a 167mm spacing where both the rear and front mount are on the same level.

195mm Spacing

The 195mm spacing is common on speed skates, where the front mount is 30mm forward in order to avoid the 2nd wheel and the front bolt from being aligned. This allows for the 2nd wheel to stick out above the frame's front mounting surface as long as the boot allows it.

Frame Height

The frame height measured is always the distance between the centre of the axles and the frame's top front mounting surface. The lowest possible theoretical height for a flat boot mounting surface is equal to the radius of the maximum wheel size. For instance 40mm height would mean that the top of 80mm wheels would be touching the skate's boot.

Frame Length

The frame length is the distance between the centre of the front and the rear axles. The shortest possible length for a frame is (n - 1) x (w + 1) where n is the number of wheels and w is the maximum wheel size. So for 4x80mm it's (4 - 1) x (80 + 1) = 243mm.