Another word for repose is closure. In music there are different kinds and degrees of closure. For instance, in music written between ca. 1400 and 1900, the most emphatic type of melodic closure tends to involve stepwise motion to the tonic scale degree -- i.e. either re-do or ti-do.
By ca. 1600, the most emphatic form of harmonic closure became the formula sol-do in the bass.
When these two formulas -- the stepwise descent to the tonic signaling melodic closure and the sol-do pattern in the bass signaling harmonic closure -- occur together, the result is a cadence which suggests a high degree of repose and an important musical arrival point. We label such a cadence a perfect authentic cadence (PAC).
If music consisted only of perfect authentic cadences, it would be rather monotonous, just as dramatic dialogue delivered in the same flat, dull tone of voice without emphasis of any sort is monotonous. Other kinds of melodic and harmonic motion, therefore, often lead to other types of cadences. These cadences suggest lesser degrees of repose or closure than a PAC does.
Consider the shape of tonal melodies: a typical tonal melody tends to descend toward its cadence.
Descending melodic motion in general tends to suggest movement from a higher to a lower state of energy, and thus of motion from a state of activity to a state of (relative) repose. This is true of earlier music as well.
Just as our ear tends to group notes together into phrases, it somtimes tends to group phrases together into longer units called periods. One common kind of period consists of two phrases of parallel construction, the second with a stronger cadence than the first. In such a period the first phrase is called the antecedent, the second phrase the consequent. Such a period is called a parallel period (See periods).
Not every pause in a musical phrase is a cadence. We call non-cadential pauses caesuras. Distinguishing between mere caesuras and true cadences in a given composition requires sensitivity to style, context and medium- to long-term continuity in the piece as a whole.
Haydn, String Quartet Op. 74 No. 3, II (first violin and cello parts only):
The Haydn melody given above is a single phrase. The caesuras in m. 2 and m. 4 articulate it into a musical sentence. These caesuras are not cadences for two main reasons. First, they led to dissonant, inverted harmonies (V# in m. 2 and vii7 in m. 4); only at the end of the passage do we attain a root-position dominant triad. Second, dissecting the phrase into two very short opening phrases plus a much longer closing phrase suggests a degree of disjunction and asymmetry that is not at all apparent when we perform or listen to the phrase. The most musically satisfying analysis is, as always, the best one.
At a cadence, two factors are notable either by their presence or by their absence: melodic closure and harmonic closure.
Melodic closure involves stepwise motion to a relatively stable, structurally consonant scale degree.
Harmonic closure involves bass motion to a relatively stable, structurally consonant scale degree, often by skip of a perfect fifth or perfect fourth.
Cadences help determine the shape of a musical composition. Some signal repose (temporary or final), while others urgently require continuation, and provide forward motion. Pay attention to the series of key centers implied by a work's cadences (its tonal form), to the ways in which a cadence leads to the next phrase (not every cadence signals a full stop and general pause -- see more under phrase rhythm), and the kinds of grouping suggested by motivic relationships between phrases (see more under periods).