In music, a catch or trick canon is a type of round - a musical composition in which two or more voices (usually at least three) repeatedly sing the same melody or sometimes slightly different melodies, beginning at different times. In a catch, the lines of lyrics interact so that a word or phrase is produced that does not appear if sung by only one voice. This phrase is often innuendo-laden, politically subversive, or lewd.
"Amongst early writers on music, the terms 'round' and 'catch' were synonymous, but at the present day the latter is generally understood to be what Hawkins (vol. ii.) defines as that species of round 'wherein, to humour some conceit in the words, the melody is broken, and the sense interrupted in one part, and caught again or supplied by another'."
"Catch originally meant simply a round for three or more voices (unaccompanied), written out at length as one continuous melody, and not in score. The catch was for each succeeding singer to take up or catch his part in time; this was evident not only from the manner in which they were printed, but also from the simple and innocent character of the words of the oldest catches, from which it would be impossible to elicit any ingenious cross-reading. But in course of time a new element was introduced into catches, and words were selected so constructed that it was possible, either by mispronunciation or by the interweaving of the words and phrases given to the different voices, to produce the most ludicrous and comical effects."
In the score for a catch the different voices are usually labelled, "1", "2", "3", etc. This indicates that voice "1" sings its part first. When the part has been completed it is typically repeated and voice "2" joins in and so on. After they have sung to the end of their parts, voice "3" joins in. Sometimes there may be variations in the words or music to be sung to each part, the second or third time it is sung. A common mistake in performance is for all parts to start together.
Noted composers of catches were Henry Purcell (A catch upon the viol ), Michael Wise (A catch upon the midnight cats ) and John Wall Callcott. Callcott's best known catch Sir John Hawkins' History of Music  ridiculed Sir John Hawkins' work by comparison with a similar work by Charles Burney. An example of a particularly lewd catch is My man John  by John Eccles.
One of the most prolific of modern composers of catches is Donald Sosin, who has written dozens of them for special occasions and a variety of vocal groups since learning the craft from his composition professor Dennis Riley at Columbia University in 1974.
Catches exhibit the property of synergy, whereby the whole is greater than the sum of the parts (and also fairly unpredictable from examination of the parts alone).
- We took off our ugly clothes
- And put on our tails again
- We combed our hair
- We're beautiful to look at
- We feel immense joy
- To be in your city
- Of course we'll tell you who we are
- We could never forgive the omission.
Which resolves into the phrase "Men's Glee Club, University of Michigan", with the second syllable of immense forming the word men's, the second syllable of ugly forming the word glee, and the first syllables of the words clothes and beautiful combining with never and city to form the word club and the word university, and so on.
Another well-known example is the "Liverpool Street Station Song", beginning, "The girl that I love has given me the shove \\ She says I am too low for her station".
Two main composition techniques are used for making the message audible: using rests in other parts that don't have a part of the message (for example, in Uitdenbogerd's "A Big Laugh" (2007), one part has "ha (rest) ha (rest)" while the other part has "(rest) don (rest) don" to make the message component "hard on, hard on"), or using the highest notes at any time to be the message melody notes. Those that rely on the second technique tend to be less obvious in performance unless emphasised. A performance technique used by Uitdenbogerd for Australian Intervarsity Choral Festival revues is to sing the catch in unison, then in parts, then with message words emphasised, and then finally only the message words. This technique only works for catches that have a message that spans the entire phrase. This method of performance has led to a new innovation in catch-writing, pioneered by M. Winikoff, in which a recognisable message melody is embedded within the catch.
Catches in popular music 
Dashboard Confessional used a catch in the chorus of the song "Hold On" from the album The Swiss Army Romance. The chorus line is "It's cruel but she's got a good hold on me", which when sung as a canon causes the message "Screw me" to be heard.
In one version of "Dona Nobis Pacem", the song can be performed as a catch. Each part of the Canon is given to one voice part and they come in at different times to create a harmony.
- Sir George Grove, ed. (1909). Grove's dictionary of music and musicians, Volume 4, p.165. The Macmillan company.
- Grove, Sir George (1904). Grove's Dictionary of Music and Musicians, p.481. Macmillan.
- A.L. Uitdenbogerd, "Sandra's Book of Rounds, Canons and Catches", , 2005