Any plainchant recitation formula, e.g. tonus lectionum (lesson tone), toni psalmorum (psalm tones; see Psalm, §II ); also the reciting note itself, by which ‘tonus’ can be synonymous with the Latin ‘tenor’.
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A church Mode , e.g. primus tonus (mode 1, or the Dorian mode), tonus authenticus, plagalis (authentic, plagal mode).