 Most authorities number the primaries descendantly , starting from the innermost primary ( the one closest to the secondaries ) and working outwards ; others number them ascendantly , from the most distal primary inwards . There are some advantages to each method . Descendant numbering follows the normal sequence of most birds ' primary moult . In addition , in the event that a species is missing the small distal 10th primary , as some passerines are , its lack does not impact the numbering of the remaining primaries . Ascendant numbering , on the other hand , allows for uniformity in the numbering of non @-@ passerine primaries , as they almost invariably have four attached to the manus regardless of how many primaries they have overall . This method is particularly useful for indicating wing formulae , as the outermost primary is the one with which the measurements begin . 
