The structure and design of a composition. Whereas in the 16th and 17th cents. instr. comps. were usually very brief (e.g. a movt. in a kbd. suite of Byrd or Purcell), by the 19th cent. they were frequently long (e.g. a sonata or sym. movt. of the later Beethoven, Bruckner, and Mahler). This implies an enormous growth in the understanding of the principles of form and in mastery of the application of those principles. In general, however, despite continuous experimentation the mus. forms so far devised can be classified into no more than 6 categories, all of them exploiting the idea of contrast plus variety both in the domain of content (thematic material) and in that of key (combinations of these are, of course, possible, e.g. in simple ternary form each section can be in binary form, and so on). (1) simple binary form (e.g. in the movts. of Bach's kbd. suites) has no strong contrast of material. The 1st section opens in the tonic key and then modulates, as it ends, into the key of the dominant (or in the case of a minor key, sometimes the relative major). The 2nd section then opens in that 2nd key and, before it ends, modulates back to the 1st. There are, then, 2 distinct main cadences, or points of rest, the 1st in the dominant (or relative major), and the 2nd in the tonic. This form, although it sometimes attained fairly considerable dimensions in the 18th cent., is unsuitable for very long pieces, since the variety offered to the listener is almost entirely confined to details of treatment and the element of key, the thematic material employed throughout being the same. This form has been little used since c.1750.(2) ternary form. This is one of the most commonly used forms for short comps. It consists of a first section (more or less complete and self‐contained), a 2nd section, contrasting as to mus. material and key (normally in the dominant or the tonic minor or relative major), and then the first section repeated. See ABA.(3) compound binary form (also known as sonata form, because often employed in the first or some other movt. or movts. of a sonata; and as first movement form for the same reason). This derives historically from simple binary form but has developed into something more resembling ternary form. Like simple binary it falls into 2 sections, of which the 1st modulates to the dominant and the 2nd takes us back to the tonic. But the sections have become elaborated as follows:1st section. Strain I (first subject) in tonic key; followed by Strain II (2nd Subject) in dominant key. Those 2 strains (or subjects) are generally contrasted in character. This section is called the exposition.2nd section. Some development (also called ‘working‐out’ or ‘free fantasia’) of the material in the previous section, followed by a repetition (recapitulation) of that section, but this time with both subjects in the tonic key so that the piece may end in the key with which it opened.Further details may incl. (a) a bridge passage, leading (in both sections) from the first subject to the second; (b) a closing passage (coda), at the end of each section.A tendency towards the evolution of simple binary form into compound binary form may be observed in some of Bach's movts., but its first real exploitation is connected with the name and fame of his son, C. P. E. Bach, and the further exploitation and elaboration with the names of Haydn, Mozart, Beethoven, and their contemporaries. This form is still in frequent use, but 20th cent. composers have modified it in detail.(4) rondo form This may be considered an extension of ternary form. If the 3 sections of that form are indicated by the formula ABA, then the rondo form must be indicated by ABACADA, or some variant of this. (The sections B, C, D, etc. are often spoken of as episodes). sonata‐rondo form, as its name implies, offers a combination of compound binary and rondo forms. The general plan is as follows: 1st section. Subject I, subject II in another key, subject I repeated. 2nd section. Development of the previous subject‐material. 3rd section. Subject I and subject II again, but the latter this time in the same key as subject I.Sometimes the development above mentioned is replaced by new material. And there are other variants.(5) air with variations. This form, which from the 16th cent. to the present day has been popular with composers of every class from the most trivial to the most serious, consists, as the name implies, of one theme (or ‘subject’), first played in its simplicity and then many times repeated with elaborations, each variation thus taking on its own individuality.There are very many types of comp. to which distinctive names are given, each representing not a ‘form’ but rather a style in which one of the above forms is presented; such as the nocturne, the gavotte, the barcarolle, the Konzertstück, and others.With the development of elec. mus. and the use of aleatory techniques in 20th‐cent. comps., the use of form is stretched to meet whatever the composer may wish to do. Infinite flexibility would seem to be the guiding principle in works of this kind.(6) See fugue.