Parallel thirds
Permitted parallel motion in thirds.
What it means
Parallel thirds are present when two voices move in the same direction while remaining continuously a third apart. Unlike parallel fifths and octaves, they are permitted in strict counterpoint and belong to the self-evident stock of polyphonic music.
Musical use
The reason for the different assessment lies in the degree of fusion. The third is an imperfect consonance: it sounds full without the two voices fusing into one, and both remain audible as independent lines. With fifths and octaves it is different — there the independence is lost, and two voices become one with doubling.
Bear in mind that parallel thirds in the diatonic scale are alternately major and minor; the interval thus stays the same by degree count, not by semitone count. Precisely this maintains the bond with the key.
Sonically, parallel thirds seem soft and unified, which is why they are omnipresent in folk music, in hymnody, and in ensemble literature — the two-voice texture of melody and lower third is the most widespread form of simple polyphony there is. Historically, the English-shaped gymel and the fifteenth-century faux-bourdon belong in this context; with them the third became established as a fully valid sonority and changed the sound of European music.
Since long chains of thirds easily become monotonous, artful settings soon switch to contrary motion or to sixths.
Parallel thirds are among the most popular devices of folk music; two-part singing in thirds is customary in many regions. In art music they give warmth to duets and wind writing. In strict counterpoint they are permitted, but should not be sustained too long, so that the voices retain their independence.
Performance practice
Play parallel thirds as a single sound: same length, same attack, same dynamics. Watch the intonation — thirds want to be taken rather low, so that the combined sound becomes calm, and in parallel chains deviations add up. And shape the line together; two differently phrased voices do not produce parallel thirds but unrest.