Hidden fifths, hidden octaves [covered fifths, covered octaves].

In part-writing, the approach to a 5th or an octave by similar motion between two parts (ex.1). Strict counterpoint permits the occurrence of hidden 5ths anywhere except between the outer parts (in which case they are sometimes called ‘exposed 5ths’). A type of hidden 5ths called ‘Horn’ fifths, however, in which each part approaches its note from an adjacent note of an overtone series containing that 5th, is permitted anywhere and in fact is common in note-against-note two-part writing.