In numismatics, 'triskeles' is singular. For example, John Melville Jones (A Dictionary of Ancient Greek Coins) defines it as “... a device formed or based upon three legs joined at the hip...”. [emphasis mine].
Wikipedia and Wiktionary considers the word to be 'triskele'. 'triskeles' is the plural of 'triskele'. This isn't merely a Wikipedia error, the Oxford English Dictionary considers the word to be 'triskele', although it undermines itself because of one its reference citations uses 'triskeles' in a singular context.
Google Books knows of 846 mentions of 'triskele' and 780 mentions of 'triskeles'. Some of the triskeles mentions are from numismatic fields, which use the word as a singular, and others are clearly using it as a plural (for example, discussing “scrolls and triskeles”.
The earliest English reference in Google Books to 'triskele' is 1868, The runic hall in the Danish old-northern museum by George Stephens. The earliest singular reference to 'triskeles' is an article on acquisitions of the British Museum in The Classical Review from 1889.
I discussed this topic in a thread on FORUM's Classical Numismatics Discussion Board where it came out that the Greek word Tri-skelês is an adjective and thus neither singular nor plural. A similar Greek word that made it into English as an adjective is 'isosceles' used to describe triangles.
I believe there are enough authorities using 'triskeles' as a singular to get it into the OED and other dictionaries as a variant spelling of 'triskele'. Certainly Barclay Head uses it as singular, and the OED cites him as an authority under 'triskeles' (for the similar word 'triskelion'). I think we have a case of the same Greek word being borrowed into English twice — with different spellings!
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