Terminology and units
X-ray emission spectra constants

Diagram lines can be designated by the arrival and departure levels of the corresponding electronic transitions, separated by a hyphen. This is the terminology recommended by IUPAC [16] and the only one currently used by CODATA. The levels in question are then designated by the capital letters K, then L, M, etc., followed by the Arabic numeral indices 1, 2, 3, etc.

However, it's convenient to continue using traditional symbols, which are much less cumbersome. These symbols were introduced by early experimenters, who were often led to attribute a recognition sign to an observed line whose origin was uncertain. On the whole, they are logical, and such terminology, codified...