They are used in the spectroscopic Siegbahn notation.

The Siegbahn notation is used in X-ray spectroscopy to name the spectral lines that are characteristic to elements.

For example, the values for and are the same for all lines (in Siegbahn notation), so the formula can be rewritten thus:

See Siegbahn notation for the newer IUPAC-recommended spectral notation system.

The K-alpha line of Moseley's time is now known to be a pair of close lines, written as (K and K) in Siegbahn notation.