In addition to the nominal size and pitch of tap, there is another important dimensional factor to be considered in selecting a ground thread tap for a given job. This is the matter of the H and L pitch diameter tap limits. H means (high) above basic pitch diameter and L (low) below basic pitch diameter. Tap limits have been established to provide a choice in the selection of the tap size best suited to produce the class of thread desired. 

The chart below illustrates the numbering system and the .0005” diameter increment separation between successive limits. Since the starting point is basic pitch diameter, dividing the limit number by 2 establishes in thousands of an inch the amount of maximum tap pitch diameter is above the basic in the H series and the amount the minimum tap pitch diameters under basic in the L series.