A musical cryptogram is a cryptogrammatic sequence of musical notes, a sequence which can be taken to refer to an extra-musical text by some 'logical' relationship, usually between note names and letters. The most common and best known examples result from composers using ciphered versions of their own or their friends' names as themes or motifs in their compositions. Much rarer is the use of music notation to encode messages for reasons of espionage or personal security (see steganography).
Because of the multitudinous ways in which notes and letters can be related, see systems below, detecting hidden ciphers and proving accurate decipherment is difficult, thus avoiding claiming the existence of inaccurate nonexistent ciphers may also be difficult.
From the initial assignment by Western music theorists of letter names to notes in the 9th century  it became possible to reverse the procedure and assign notes to the letters of names. However, this does not seem to have become a recognized technique until the Baroque period. From the mid-19th century it has become quite common. Sporadic earlier encipherments used solmization syllables.
Syllables to solmization names
It is believed that this method was first used by Josquin des Prez in his Missa Hercules Dux Ferrarie. It was named Soggetto cavato by the later theorist Zarlino. Under this scheme the vowel sounds in the text are matched to the vowel sounds of the solmization syllables of Guido of Arezzo (where 'ut' is the root, which we now call 'do'). Thus the Latin name of the dedicatee 'Hercules Dux Ferrarie' (Ercole d'Este, Duke of Ferrara) becomes re-ut-re-ut-re-fa-mi-re, which translates as D-C-D-C-D-F-E-D in modern notation with C as 'ut'. This is used as the cantus firmus of the mass setting. Josquin's method was imitated by several of his contemporaries and successors, including Adrian Willaert and Costanzo Festa (see the article on Soggetto cavato).
Letters to note names
Since the note names only cover letters A to G (reflecting the octave repetition of these names), the problem arises as to how to cipher the rest of the alphabet. Historically there have been two main solutions, which may be labelled for convenience the 'German' and the 'French' methods.
Because the development of note names took place within the framework of modes, in the German-speaking world B-flat was named 'B' and B-natural was named 'H' (see the entry for musical Note). The most common musical cryptogram is the B-A-C-H motif, which was used by Johann Sebastian Bach himself, by his contemporaries and by many later composers. Other note names were derived by sound, for example E-flat, 'Es' in German, could represent 'S' and A-flat the digraph 'As'. Composers less fortunate than Bach usually seem to have chosen to ignore non-musical letters in generating their motifs. For example Robert Schumann, an inveterate user of cryptograms, has just S-C-H-A (E-flat, C, B-natural, A) to represent himself in Carnaval. Sometimes phonetic substitution could be used, Schumann representing Bezeth by B-E-S-E-D-H. Johannes Brahms used B-A-H-S (B-flat, A, B-natural, E-flat) for his surname in the A-flat minor organ fugue, and the mixed language Gis-E-La (G-sharp, E, A) for Gisela von Arnim, among many examples.
The 'French' method of generating cryptograms arose late in the 19th century and was more akin to normal encipherment. The most popular version involved writing out the letters H-N, O-U and V-Z in lines under the original diatonic notes A-G, as follows:
so that A, H, O, and V are enciphered by note 'A', B, I, P and W by 'B' (flat or natural) and so on. This scheme was used by Jules Ecorcheville, editor of the journal S.I.M., to solicit centenary commemorations of Joseph Haydn in 1909, except that he diverted the 'H' to B-natural, presumably to avoid too many repeated notes. Writing to Gabriel Fauré about the invitation, Camille Saint-Saëns said he was writing to Ecorcheville asking him to prove that Y and N could signify D and G as "it would be annoying to get mixed up in a farcical business which would make us a laughing stock in the German musical world."  The many-to-one mapping of this method makes it more difficult to extract possible motifs from the musical score than the one-to-one correspondence (apart from 'As') of the German system.
A French tradition of celebratory uses developed from the Haydn centenary, with tributes to Gabriel Fauré by Maurice Ravel, Florent Schmitt, Charles Koechlin and others in 1922 (added to later by Arnold Bax, 1949) and to Albert Roussel by Francis Poulenc, Arthur Honegger, Darius Milhaud and others (using various ciphering schemes) in 1929. Honegger's system involved placing the letters after 'H' under sharpened and flattened notes, an example of how chromatic cryptograms could be more easily accommodated in 20th-century music.
Dmitri Shostakovich used the German scheme for his personal motto D-Es-C-H (D, E-flat, C, B-natural), representing D.SCH, which appears in many of his most characteristic works. Elliott Carter featured both a cryptogram for the last name "Boulez" in his piece Réflexions (2004) and a sonic symbol of the first name "Pierre".
Cryptograms were less common in England, but Edward Elgar, who was also interested in general cryptography and puzzles, wrote an early Allegretto for his pupils the Gedge sisters using G-E-D-G-E  and part of the 'enigma' in the Enigma Variations involves cryptograms.
In 1947 Friedrich Smend also been suggested that Bach enciphered significant numbers through methods including repetitions of a motif, word, or phrase; the notes played on the continuo; the use of sequence; and the notes played by the accompaniment. However, Ruth Tatlow has presented evidence questioning the plausibility of Smend's claims.
Summary of signature motifs
The following list includes only motifs which are known to have been used in published works.
- A, B, E, B, A
- for Aubrey Brain, used in Gordon Jacob's Sextet for Piano and Winds, which was inscribed "In memoriam Aubrey Brain"
- A, B♭, B♮, F (= A, B, H, F)
- A, B♭, E, G, G (= A, B, E, G, G)
- A, D, A, A, F (= A, L, A, I, N)
- for Jehan Alain, used by Maurice Duruflé in his Prélude et Fuge sur le nom d'Alain (op. 7) and derived on the French system but leaving H = B-natural and starting the second line with 'I'
- A, E♭, C, B♮ (= A, S, C, H) and A♭, C, B♮ (= As, C, H)
- used in Schumann's Carnaval, Op. 9. He was romantically involved with Ernestine von Fricken, who came from the town of Aš, in German "Asch". Every piece in the whole cycle is based on one or other of these motifs. The letters are also the note names from 'Schumann' and he added the motif E♭, C, B, A (= S, C, H, A) so that three are displayed in breves in the section 'Sphinxes'. It has been noted that ASCH and SCHA also appear in the title of Schumann's Faschingsschwank aus Wien, Op. 26.
- [A], E♭, C, B♮, B♭, E(♮), G (= [A], S, C, H, B, E, G)
- B♭, A, B, E♭ (= B, r, A, H, m, Es)
- for Brahms, used by Alfred Schnittke in his Quasi Una Sonata along with the B-A-C-H motif and other quotations, and allusions
- B♭, A, C, B♮ (= B, A, C, H)
- B, A, D, D, G (= H, A, Y, D, N)
- for Joseph Haydn, used by Maurice Ravel in his Menuet sur le nom d'Haydn and by other contributors to the S.I.M. commemoration and derived on the French system
- B, E, B, A or B, A, B, E
- for Béla Bartók (Béla Bartók, the latter motif recognizing the Hungarian practice of placing the family name before the personal name, see eastern order)
- C, A, G, E
- for John Cage, used by Pauline Oliveros and, in the composition CAGE DEAD, by Simon Jeffes of the Penguin Cafe Orchestra
- D, E♭, C, B (= D, S, C, H)
- for Dmitri Shostakovich (D. Schostakowitsch)
- E♭, A, C, B, E, D (= Es, A, C, H, E, Re)
- E, A, E, D, A (= E, La, Mi, Re, A)
- E♭, C, B♮, A (= S, C, H, A)
- F, A, E and F, A, F
- for Frei aber einsam and Frei aber froh, "free but lonely" and "free but happy" in German; the former, his friend Joseph Joachim's motto (F-A-E Sonata), described as "more romantic" than the latter, a "gender-separatist" motto of Johannes Brahms's Symphony No. 3
- F, E♭, C, B (= F, S, C, H)
- G, A, B, D, B, E, E, F, A, G, D, E (= G, A, B, Re, H, E, Le - F, A, Ug, Re, E)
- for Gabriel Fauré, used by contributors to Henri Prunières' Fauré celebration in the October 1922 issue of La revue musicale  (F, A, G, D had also been used 20 years earlier for "Fauré" in a collaborative string quartet)
- G, A, D, E
- for Niels Gade; the motif is the basis of Robert Schumann's "Nordisches Lied", No. 41 from Album for the Young, Op. 68. Its subtitle is "Gruss an G."
- Hiley, David 'Notation III, 1 (vi)' in Sadie, Stanley (ed.), The New Grove dictionary of music and musicians, Macmillan, 1980, (6th ed. of the Grove dictionary), vol.13, p. 348–349
- Sams, Eric 'Cryptography, musical' in Sadie, Stanley (ed.), The New Grove dictionary of music and musicians, Macmillan, 1980, (6th ed. of the Grove dictionary), vol.5, p. 80
- quoted in Nichols, Roger, Ravel, Dent, 1977, ISBN 0-460-03146-5, page 66
- David Parlett, ed. "Baxworks", Music by Arnold Bax.
- Halbreich, C., 'Ciphered creed', Music and musicians, vol. 20 (1972) p. 18
- Justin Wintle (2009). The Concise Makers of Modern Culture, p.123. ISBN 0-415-47782-4.
- McVeagh, Diana, Elgar the music maker, Boydell Press, 2007, ISBN 978-1-84383-295-9, p. 3
- Tatlow, Ruth (1991). Bach and the Riddle of the Number Alphabet, p.25. ISBN 0-521-36191-5.
- Taruskin, Richard (2009). Music in the Early Twentieth Century, p.324. ISBN 0-19-538484-9. "the 'Eschbeg set,' Schoenberg's musical signature." and "the 'A. Schbeg' (or 'Aschbeg') set."
- Christopher Alan Reynolds (2003). Motives for Allusion: Context and Content in Nineteenth-Century Music, p.31. ISBN 0-674-01037-X.
- Whittall, Arnold. 2008. The Cambridge Introduction to Serialism, p.206. Cambridge Introductions to Music. New York: Cambridge University Press. ISBN 978-0-521-86341-4 (hardback) ISBN 978-0-521-68200-8 (pbk).
- Reel, James (2010). "Lyric Suite", AllMusic.com.
- Blom, Eric (1961). Some Great Composers, p.85. ISBN 0-19-315133-2.
- Schmelz, Peter J. (2009). Such Freedom, If Only Musical, p.254. ISBN 0-19-534193-7.
- "Dear.John: A Canon on the Name of Cage", Larry Polansky, BMARS.
- Adam Krims, ed. (1998). Music and Ideology: Resisting the Aesthetic, p.Z-69. ISBN 90-5701-321-5.
- Nectoux, Jean-Michel (translated by Roger Nichols), Gabriel Fauré: a musical life, Cambridge University Press, 1991, ISBN 0-521-23524-3, p. 427
- Ondine Tobin Young (1996). Frei aber einsam, frei aber froh: cyphered motives and performance practice. University of California, Santa Cruz.