from Edward Bunting, The Ancient Music of Ireland (Dublin 1840), page 21: The strings of the harp.
Irish téad an leathghléis
spoken by Gráinne Yeats
Scottish Gaelic teud an leth-ghleus spoken by Tony Dilworth
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Tead a' leithghleas ('in English characters' Tead a leith glass) - String of the half note
Proper spelling may be téad an leathghléis. Leath means 'half'. Gléas (genitive case gléis) has a range of applications including 'order, arrangement; means, provision; instrument, apparatus, tackle; a key (in music)'; gléas ceoil means 'a musical instrument'. The first tune in the insert following p.88 (p.1) is entitled "Feaghan Geleash - or Try if it is in tune" (spelt Feaghan Gleash on p.82), where the Gaelic words are probably féachain gléis, 'a test of tuning': thus gléas may also mean 'tuning', an extension of the meaning 'order, arrangement'. On p.23 Bunting gives us the 'Scale of the Irish Harp of thirty strings, tuned in the natural key, termed, "Leath Gleas," or half note', and here again tuning is involved. There is considerable evidence for gléas as 'tuning' even from medieval literature (as O'Curry argues, Manners and Customs III, p.255; see also Ceol Tíre 25.5), but there appears to be no support for Bunting's equating gléas with 'note'. See also Leithghleas below. In a footnote on p.23 he mentions 'the natural key termed "Leath Glass"', but the noun glas normally means 'a lock' (see Glas below) and there is nothing I can see to justify its use for 'a note'. We might note its occurrence in the names of some Scottish tunes, not least A' Ghlas-mheur - The Finger-lock (see Scottish Gaelic Studies XIX :191.11).
Colm Ó Baoill 2002
The significance of this string is obviously that its tuning defines which of the two main tunings the harp is set in. By turning it to f# as indicated, (and then setting its octaves also to f#) the harp is set in the ‘sharp or natural tuning’, that is the natural major scale of G, called by Bunting Leithghleas.
However if this string is turned down to f natural, as indicated in the ‘flatt or high bass key’ the harp is set into a mixolydian scale of G, that is G major but with a flattened 7th. Bunting gives us the term Fuigheall beag for this tuning, but we may wonder if there is a distinct name for this string when it is set to f natural.
See also Alasdair Codona's comments on this string.
Simon Chadwick 2008