Johann Georg Sulzer, "Phrase," General Theory of the Fine Arts [Allgemeine Theorie der schönen Künste], vol. 2 (1774), 35-36.

Phrase [Einschnitt]. The names that are attached to the larger and smaller segments of a melody are, up to now, still somewhat indefinite. We speak of periods [Perioden], sections [Abschnitten], phrases [Einschnitten], rhythms [Rhythmen], caesuras [Cäsuren], etc., in a way such that the same word occasionally has two meanings, or two different words occasionally have the same meaning. In this work, we will call those main segments of a melody that begin with a new tone and end with a perfect cadence Periods or Sections. They will be discussed in a separate article (cf. "Period"). The smaller segments that generally constitute the period, and are generally called rhythms, we will call Phrases. The smaller segments, brought about by brief pauses in the middle of phrases, we will call Caesuras. According to this nomenclature, a melody consists of periods, periods of phrases, [and] the phrases (when they are not simple) of caesuras.

Phrases in a melody [Gesang] are what the verse is in poetry. Each consists of a short series of exactly ordered tones, which the ear can group together and grasp at once as an indivisible segment. They must be designed such that there is no stop at any tone, nor a feeling of a pause, until we reach the last tone, at which the ear feels a noticeable break [Abfall].

Both [of these characteristics, perceptual immediacy and melodic continuity] are achieved by avoiding perfect consonances in the melody and triads in the harmony in the middle of the segment, at whose end, though, a small pause is made apparent by means of such consonances, or the triad, or also by means of cadences.

Since a phrase must be grasped at once as a single segment, it cannot exceed, therefore, a certain length; for at its end, its beginning must not yet be extinguished from our ear. In poetry, the longest verse is six feet because it was noticed that the ear cannot grasp any more feet at one time. The longest phrases of a melody are five, at most seven, measures long, and in this [latter] case they must have caesuras, just as do longer verses [in poetry]. The shortest verses have two feet, and the shortest phrases have two measures. Just as a series of many such short verses would soon become monotonous, however, so a melody of such short phrases would have no appeal. Those of four measures are the most common and best. We can also make them three measures long. If they are to sound good, though, two segments of three measures should always be linked so that, as phrases of six measures, they are felt as having a caesura in the middle. These [six-measure phrases] are suitable for triple meters.

Insofar as we consider merely the pleasant sound of the melody, phrases of equal length throughout the melody are the best. And such is the case in all dance melodies. Where some special expression of feeling is to be achieved, individual phrases that are longer or shorter than otherwise in the piece have a good effect.

Some foresight is required in order to design a melody so that, with respect to rhythmic arrangement, the ear is never offended. A comprehensive discussion about this would be too vast for this work and can rightly be passed over here since this matter has recently been more thoroughly discussed by a master of the art, to whom I refer the musical amateur.*

* Johann Phillip Kirnberger, The Art of Strict Musical Composition, 403-17.