Keay, John; Keay, Julia (1994). Collins Encyclopaedia of Scotland. Harper Collins. s. 405. ISBN0002550822. «GAELIC PSALM SINGING A specific kind of singing in the Gaelic Church in which the psalm tune is "lined out" by a precentor, Gaelic psalm singing is performed slowly in unison or octaves by the congregation with much decoration and with no attempt to produce a uniform choral result. The tunes are post-Reformation, but the style, which transforms the melody, shows local variants and is generally agreed to reflect much older vocal practices; known as 'free heterophony', it finds its nearest parallels among the Copts in the Middle East.»
Temperley, Nicholas (1981). «The Old Way of Singing: Its Origins and Development». Journal of the American Musicological Society34 (3): 511–544. JSTOR831191. doi:10.2307/831191.
Temperley, Nicholas (1981). «The Old Way of Singing: Its Origins and Development». Journal of the American Musicological Society34 (3): 511–544. JSTOR831191. doi:10.2307/831191.
