PartiTA n0. 2 IN D MINOR, BWV 1004
From the very earliest dance collections, even before they were written down, it was the custom to play dances in pairs, with the second one being a varied repeat of the first at double the speed. Later, dances were collected into suites (or partitas) from volumes of like-dances, by court musicians. For example, an allemande would be chosen from a collection of allemandes and then a courante from a collection of courantes and so on, while the kapellmeister attempted to find thematic connections that would unite the dances. Later still, and in Bach's time, a composer would write a suite as we now know it, often interweaving recurring motivic ideas or compositional devices (though without necessarily quoting other music directly), to create unity throughout the work on a large scale.
In the second Partita, for example, there is an underlying harmonic progression based on a descending tetrachord, that is interwoven in the entire work —and that foreshadows the beloved Ciaccona. Also, Bach unifies the movements with the importance that he gives to the sixth scale degree, B-flat.
Considering only the opening phrases of the movements, the Allemanda moves from the tonic up to B-flat and then leaps down to the following bass note, and again back to B-flat. The Corrente's first moving line begins its descent from B-flat, and the Sarabanda and Ciaccona could both be interpreted as beginning with one of the inner voices moving from A to B-flat and back to A. The Giga, after establishing the key of D minor, begins its harmonic movement by leaping to a high B-flat, and then through a circle of fifths, winds its way back down to the tonic.
Looking at the second partita in those terms, we see thematic connections both in the bass and in the various contrapuntal lines throughout the movements. Indeed, the closing phrase of the Ciaccona is a motivic answer to the opening of the Allemanda; and thus, having experienced all affekts from elation to profound sorrow, we have come full circle and Bach brings this monumental and much loved work to a close.