It is one of the most famous pieces of American music — but for 70 years orchestras may have been playing one of its best-known effects wrong.

The work is George Gershwin’s jaunty, jazzy symphonic poem “An American in Paris,” and the effect involves a set of instruments that were decidedly not standard equipment when it was written in 1928: French taxi horns, which honk in several places as the music evokes the urban soundscape that a Yankee tourist experiences while exploring the City of Light.

The question is what notes should those taxi horns play. In something of a musicological bombshell, a coming critical edition of the works of George and Ira Gershwin being prepared at the University of Michigan will argue that the now-standard horn pitches — heard in the classic 1951 movie musical with Gene Kelly, in leading concert halls around the world, and eight times a week on Broadway in Christopher Wheeldon’s acclaimed stage adaptation — are not what Gershwin intended.