With respect to their permanent successors, the sum of the mesio-distal diameters of the first and second deciduous molars is generally:

Leeway space Leeway space is the difference between the sum of the mesiodistal widths of the primary canine, first and second molars to permanent canine, first and second premolars. The unerupted canine, premolars are on an average 1.7 mm smaller per quadrant in the lower arch and 0.9 mm per quadrant in the upper arch.