When the _sonata_ has four movements, these are usually arranged as follows:
1. A quick movement (_allegro_, _presto_, etc.), often preceded by a slower introduction.
2. A slow movement (_largo_, _andante_, _adagio_, etc.).
3. A minuet or scherzo, often with a trio added, in which case the part preceding the trio is repeated after the trio is played.
4. A quick movement--the finale, sometimes a rondo, sometimes another sonata-form, sometimes a theme with variations.
These movements are all in closely related keys, but in a variety of contrasting rhythms.
154. A _trio_ is a sonata for three instruments (such as piano, violin, and cello), while a _quartet_ is a sonata for four instruments, the most common quartet combination being as follows: First and second violins, viola, and violoncello.
The term _chamber music_ is often applied to instrumental music for trio, quartet, quintet, and other similar combinations which are suitable for a small room rather than for a large concert hall.
The words _trio_ and _quartet_ are also applied to vocal works for three and four voices respectively, these having no relation whatsoever to the sonata as described above. The word _trio_ is also applied to the middle section of minuets, scherzas, marches, etc., the term originating in the old usage of writing this part for three instruments only.
155. A _concerto_ is a sonata for a solo instrument with orchestral accompaniment, the form being usually somewhat modified so as to adapt it to a composition in which there must necessarily be opportunity for a good deal of technical display. There are usually but three movements in the _concerto_.
The great majority of _concertos_ are for piano and orchestra, but examples of concertos for violin, cello, flute, oboe, and other solo instruments (all with orchestral accompaniment) have also been written. A few modern composers have applied the term _concerto_ to certain large organ works (with no orchestral accompaniment, the composition being written for just the one instrument), but this use of the word is so contrary to the accepted definition that it is hardly justifiable.
When a concerto is played on two pianos (without orchestra), this does not mean that there is no orchestral part, but that there is no orchestra to play it, and so the parts that should be played by the orchestral instruments have simply been arranged for a second piano (sometimes organ).