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"Thoroughbass methods recognized that sometimes it was appropriate to add a sixth to a diminshed fifth (when the diminshed triad functions as a leading-tone chord) and sometimes it was inappropriate to do so (when the diminished triad functions as a supertonic chord in minor). To facilitate making this distinction, Georg Philipp Telemann proposes placing an arc (a fermata without a dot) over a diminished fifth which did not permit a minor sixth to be played (a supertonic chord in minor), and C.P.E. Bach uses this notation consistently in his 1762 thoroughbass treatise." Joel Lester, Compositional Theory in the Eighteenth Century, p. 243.