The smallest structure in music is a figure, sometimes known as a cell. The melody is made mainly from repetitions of a four-note figure at different pitches.
This is the simplest form, using the same tun for each verse of a song, as in a traditional blues or a Christmas carol. If a strophic song has three verses, the structure could be described as AAA
In verse and chorus form, each verse (A) has similar music but different words, and is followed by a contrasting chorus (B) in which both the words and the music are usually the same every time. The chorus may be repeated to make it all the more memorable.
A B A B B
Binary form has two sections, each usually repeated, in the pattern A A B B. The second section is usually longer than the first, and the sections are often not very contrasted in content. The most important feature is that section A ends in a related key (the dominant if the main key is major, or the relative major if the main key is minor), while section B ends in the tonic. Binary form was often used for dances in the 17th and 18th centuries, and for short piano pieces in the 19th century.
Ternary form has three sections, the first of which returns at the end, either exactly (A B A) or varied (A B A1). The B section usually contrasts with the A sections in key or content (or both). The repeat of the first section may be indicated by the term da capo at the end of the middle section, avoiding the need to write out the first section again. Many 18th century songs do this and are consequently known as 'da capo' arias,.
Sonata form begins with a section called exposition in which a main theme called the first subject is heard in the tonic key. The music then moves to a closely related key, which often features a contrasting second subject in the new key. This material is transformed in various ways, and in a variety of keys, in a central section called the development. In the final section, called the recapitulation, material from the exposition returns, now al the tonic key. The movement may end with a coda and sometimes there is a slow introduction before the start of the exposition.
Rondo form is based on the idea of a rondo theme (A) that repeatedly comes around in the tonic key between contrasting episode (B, C and so on) in related keys. This gives rise to structures such as A B A C A or A B A C A D A
Arch form has a symmetrical structure such as A B C B A
The 'ritornello' of the title refers to the opening section in the tonic key. Parts of the ritornello then return in related keys, separated by modulating episodes for one of more soloists. The final ritornello is in the tonic key. This differs from rondo form in that a rondo theme is normally complete and in the tonic every time it returns.
Suite - before 1750 the suite was a collection of short dances, mostly in binary form, sometimes preceded by a longer introductory movement entitled overture or prelude
Concerto - a type of music in which a soloist (or small group of soloists) is heard in contrast to, and in combination with an orchestra. Most concertos has three movements, in the order fast - slow - fast.
Symphony - the most common form of orchestral music from around 1750. Typically in four movements, in the order fast - slow - minuet - fast. The minuet was originally a dance in triple-time. It was replaces by the faster 'scherzo' in the 19th century. The slow movement might be in ternary form or theme and variations. The last movement might be in rondo form, sonata form or a combination of the two.
Popular song form developed from verse and chorus songs in the early 20th century A short introduction replaces the first verse and the rest of the song consists of a repeated 32 bar chorus made from four eight bar phrases in the pattern A A B A. Each phrase has different lyrics, but the A phrases all have the same chord pattern and melody, sometimes based on a hook - a short, repeated melodic idea. The B phrase, known as the 'middle eight' or 'bridge' has contrasting tune with different harmonies, and lyrics that generally offer a different slant on the idea expressed in the A phrases.
Pop songs since the middle of the 20th century use a variety of structures but often have an extended verse and chorus form, in which there may be an introduction at the start, an instrumental that substitutes for one of the sung verses, and a coda or outro to provide a conclusion. The chorus may have an A A B A structure, particularly in more old-fashioned songs. There may also be a pre-chorus a short section that elads into the chorus proper, set to the same words each time it comes around.