I always thought it was a question of nationality: English = Diapason; Continental (French) = Bourdon.
For English seventeenth-century usage: Thomas Mace, Musick's Monument, 1676, in discussing the Theorbo (Chapt. XLII-XLIII pp. 207-230), mentions Diapasons often, as, for example in the following (p. 218; when he states 26 strings, he means 13 pairs): "Therefore I say, a Theorboe cannot be Compleat, if it have not 26 Strings; so, as that from the Gam-ut String [i.e., the sixth course], there may be a perfect Gradual Descent of a Compleat Eighth in Diapasons... ." pp. 220-221 he gives a musical example in tablature where he adds the extra course below the sixth to various fingerings of F chords, as an extra deep bass, stating, " that the Addition of One Diapason has given you above a Double Number of Stops..." [meaning the sixth course can be used for the notes A and C, and the fifth course as For open C, over the low F, not possible when the lowest available course is the sixth as G]. Earlier, pp. 86-87, in the Lute section of his book, after exercises on each of the first six courses, he has an extra exercise that uses the additional bass strings below the sixth course. This exercise, last on p. 87, is identical to the second on p. 86, except the first, p. 86, has letters only on the second course, while that on p. 87 has extra letters and numbers under the sixth course (5, ///a, 4, etc.). Of this he states, " Therefore, take notice, of This next Musick Line; which is the very same I set you a little before, only I have added to It, some Basses, or Diapasons...." Viz: the first begins: -----------|-----------|-----------|- a-b-d-b-|-a-f-d-b-|-a-d-f-h-|- [etc.] -----------|-----------|-----------|- -----------|-----------|-----------|- -----------|-----------|-----------|- -----------|-----------|-----------|- The second, with the "Basses, or Diapasons" begins: -----------|-----------|-----------|- a-b-d-b-|-a-f-d-b-|-a-d-f-h-|- [etc.] -----------|-----------|-----------|- -----------|-----------|-----------|- |----------|-|----------|-|----|----|- -----------|-----------|-----------|- 5 5 ///a 4 GJC
