Incorrect: He is the principle musician in the band.Principle is always a noun, which is sometimes erroneously used with the meaning of the adjective principal. Principal should not be confused with principle. ( obsolete, Latinism ) Of or relating to a prince princely.
In theory, there are the same number of principal components as there are variables, but in practice, usually only a few of the principal components need to be identified to account for most of the data variance. Colowick, Alan Hall (editors), Methods in Enzymology, Volume 403, page 22, Collins, Application of Phylogenetic Algorithms to Assess Rab Functional Relationships, Sidney P. inscribed with Hammurabi′s laws written in Akkadian in cuneiform script. The principal treasure of ths department, however, is the Stele of Hammurabi (1792-1750 B.C.), king of the first Babylonian kingdom, a basalt cylinder 2.25m/7ft 5in. 1995, Madeleine Cabos, Baedeker Paris, page 105,.In a word, the Epiſodes of Homer are complete Epiſodes they are proper to the ſubject, because they are drawn from the ground of the fable they are ſo joined to the principal action, that one is the neceſſary conſequence of the other, either truly or probably: and laſtly, they are imperfect members which do not make a complete and finiſhed body for an Epiſode that makes a complete action, cannot be part of a principal action as is eſſential to all Epiſodes. Alexander Pope, The Odyssey, Volume 2, page 217,
The principal cause of the failure was poor planning.