Arch form

What it means

Arch form is a symmetrical design in which the sections return in reverse order after a midpoint. The basic scheme is ABCBA: the central section C forms the mirror axis, and around it B and B correspond, as do A and A. The simpler form ABA, the ternary song form, can be understood as the smallest arch.

Musical use

The effect rests on the feeling of return. After the centre, the music passes through the stations it already knows once more, now in reverse order, and closes where it began. This produces closure and balance, especially when the middle section forms a clear contrast or climax.

The return need not be literal. Often the corresponding sections appear in altered form — shortened, varied, differently orchestrated — so that the symmetry remains recognizable as a framework without seeming rigid. Arch forms can refer to the arrangement of the movements of a multi-movement work as well as to the construction of a single movement.

In the twentieth century, arch form became a favoured structural principle, also in combination with other forms of symmetry such as retrograde and mirror. But the principle reaches far back and is found wherever a piece returns to its beginning.

Arch form is to be distinguished from rondo, in which a refrain returns several times with changing episodes and no mirroring is built in, and from bar form (AAB).

Arch form occurs on different scales. On the small scale, a single movement can be built on the scheme ABA or ABCBA; on the large scale, the movements of a multi-movement work can be arranged symmetrically around a central movement. Béla Bartók in particular used the design in string quartets and orchestral works. The symmetry is rarely rigid: the returning sections usually appear altered and shortened.

Performance practice

Make the design clear to yourself before practising and mark the correspondences. Shape the returning sections so that the return becomes audible, but with the knowledge of what happened in between. And give the centre its weight — it is the point around which everything is ordered.