The Upanishads, Part 2 (SBE15), by Max Müller, , at sacred-texts.com
1. Om 1! Let us explain Sîkshâ, the doctrine of pronunciation, viz. letter, accent, quantity, effort (in the formation of letters), modulation, and union of letters (sandhi). This is the lecture on Sîkshâ.
46:1 Cf. Rig-veda-prâtisâkhya, ed. M. M., p. iv seq.