Bar form (German: die Barform or der Bar) is a musical form of the pattern AAB.
Original use 
The term comes from the rigorous terminology of the Meistersinger guilds of the 15th to 18th century who used it to refer to their songs and the songs of the predecessors, the minnesingers of the 12th to 14th century. In their work, a Bar is not a single stanza (which they called a Liet or Gesätz); rather, it is the whole song. The word Bar is most likely a shortening of Barat, denoting a skillful thrust in fencing. The term was used to refer to a particularly artful song – the type one composes in songwriters' guilds.
The AAB pattern does, however, describe each stanza in a Meistersinger's Bar, which is divided into two Stollen (A), which are collectively termed the Aufgesang, followed by an Abgesang. The musical form thus contains two repetitions of one melody (Stollen - 'stanzas') followed by a different melody (Abgesang - 'aftersong'). One such tune (Ton in Meistersinger terminology) by Hans Folz (c1437-1513) illustrates this:
Note that the B section is not necessarily the same length as each A section. The B section can also incorporate parts of the A section's phrase: in the above example, the final 14 notes of the B section match the final 14 notes of each A section (see also Rundkanzone). In this example, the 17 never-repeated notes starting the B section would have been called a Steg by the Meistersingers: literally, "bridge"; whence comes the term for a contrasting section in popular music.
Modern use 
Composer Richard Wagner in act III of his opera Die Meistersinger von Nürnberg, used the word Bar incorrectly as referring to a stanza of the prize song. This was based on his misreading of Wagenseil. In addition, Bach's famous biographer Spitta in his monumental 1873-1880 biography, emphasized the role of Lutheran chorales, almost all of which are in AAB form, in what he considered the most mature of Bach's cantatas. Composer Johannes Brahms claimed the AAB form of the chorale "Jesu meine Freude" generates larger formal structures in Bach's motet of the same name. Subsequent popularity and study of the use of AAB stanzas in Bach's and Wagner's works has led to wide adoption of the term Bar form for any song or larger musical form that can be rationalized to a three part AAB form with the first part repeating.
Such AAB forms may be found in works ranging from Lutheran chorales to "The Star-Spangled Banner" to songs by Schubert, Schumann, and Brahms. Bartok made use of the Bar form in the 20th Century, and most blues follow the pattern "AAB."
- Harvard Concise Dictionary of Music, entries on Bar form and minnesingers. (ISBN 0-674-37471-1)
- Encyclopædia Britannica (2005), article on Bar form. 
- A History of Western Music by Donald Grout (ISBN 0-393-09416-2)
- Wagenseil, Johann Christoph (1697). De sacri rom. imperii libera civitate noribrgensi commentatio, appendix Buch von der Meister-Singer Holdseligen Kunst. Altdorf.
- Spitta, Phillip (1873-1880). Johann Sebastian Bach, 2 vols. Leipzig: Breitkopf & Härtel.