Pale crag martin to do so, it would cast doubts on their specific distinctness. There are several subspecies differing in plumage shade or size, although the differences are clinal, and races interbreed where their ranges meet. The pale crag martin of the nominate subspecies "P. o. obsoleta" is 12–13 cm (4.7–5.1 in) long with light brown upperparts, becoming paler on the lower back, and a short square tail that has small white patches near the tips of all but the central and outermost pairs of feathers. It has a pale grey throat, upper breast and underwing coverts, and the rest of the underparts
A Whiter Shade of Pale "The Miller's Tale"). Thus, this piece can be considered an early example of progressive rock. The phrase "a whiter shade of pale" has since gained widespread use in the English language, noticed by several dictionaries. As such, the phrase is today often used in contexts independent of any consideration of the song. It has also been heavily paraphrased, in forms like "an Xer shade of Y", to the extent that it has been recognised as a snowclone – a type of cliché and phrasal template. The song is in moderate time in C major and is characterised by the bassline