Numbering Conventions
In order to make the discussion of such topics as moult processes or body structure easier, ornithologists assign a number to each flight feather. By convention, the numbers assigned to primary feathers always start with the letter P (P1, P2, P3, etc.), those of secondaries with the letter S, those of tertials with T and those of rectrices with R.
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.
Secondaries are always numbered ascendantly, starting with the outermost secondary (the one closest to the primaries) and working inwards. Tertials are also numbered ascendantly, but in this case, the numbers continue on consecutively from that given to the last secondary (e.g. … S5, S6, T7, T8, … etc.).
Rectrices are always numbered from the centermost pair outwards in both directions.
Read more about this topic: Flight Feather
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