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Transposition is the process of changing written music from one key to another. To transpose songs, a musician must have a basic understanding of interval relationships and key signatures. The actual process consists of three major steps, including analyzing the key of both the instrument and the original music, looking at pitch intervals note by note and checking any accidentals.
The first step to transpose songs is to figure out the key in which the instrument that will play the music performs. A clarinet, for example, must play a B flat in order to match a C on the piano. The key of these instruments determines how far a musician must adjust for performance. Some instruments such as violins, flutes and oboes do not transpose, meaning they play a C to match a C on the piano. Transposing for these instruments is done only to accommodate range problems or to switch to a key that is not as difficult for the performer.
Once musicians know the key of the instrument for which they must transpose songs, they look at the key in which the music is originally written. They then compare that to the key in which they want the music to be. For transposing instruments, this usually is the key in which they play, unless there are range difficulties.
As an example, suppose the original music was in the key of C. The musician, however, needs the music to be in the key of F, either because that is the way his instrument is pitched, or because the key of C creates performance issues. The key of F has one flat, B flat, so the musician uses this as the new key signature and treats F as his "home base" note, or tonic. He then recognizes that the interval between C and F is a perfect fourth going up or a fifth going down. This tells him that he must raise all notes in the original music by a perfect fourth — or alternately, lower them a fifth — in order to get them in the key of F.
As the musician raises or lowers the individual pitches of the music to accommodate the new key, he follows a basic rule: Transposition does not change the mode or major or minor assignment of the work. A piece that is in major, for instance, remains in major. This happens because the musician doesn't change the written melodic interval relationships or what scale degrees the pitches represent in the original key. He merely changes the starting point around which the intervals center.
During the transposition process, the musician takes care to focus on any accidentals, or altered pitches, in the piece, as it is the interval, not the accidental symbol, that must remain constant. For example, a diatonic, or major scale, consists of the following interval series: whole step, whole step, half step, whole step, whole step, whole step, half step. In the key of C, the pitches of the related diatonic scale thus are C, D, E, F, G, A, and B, so an augmented, or raised, fourth starting at C would require an F sharp. In the key of F, the pitches of the related diatonic scale are F, G, A, B flat, C, D and E, so an augmented fourth would require a B natural.
Although musicians with basic music theory knowledge can transpose songs fairly easily manually, doing so is time consuming. If a musician does not have time to transpose songs, he can enter the music in the original key quickly into various music software programs. With these programs, the musician can select the key in which he wants the music to be and the computer, via the program, will do all the transposition work automatically. The advantage of doing this is that, should the musician need the same piece in multiple keys, he can enter a few quick commands in the program to get what he needs instead of going through the entire piece note by note for every key required.
I have seen specialized pianos, mostly used by songwriters like George Gershwin, that would transpose songs by shifting the entire piano frame one direction or another. In other words, if the piano were in its original C configuration and the pianist wanted to play a song in the key of E flat, a special knob would move the strings down several steps.
Instead of having to look at sheet music written in C and played in E flat, the pianist could use the same key of C fingerings but the singer would be pitched in E flat. Most professional musicians can transpose music on sight if a singer needs to raise or lower the overall pitch, but these pianos were popular with song composers who preferred to work in simpler keys like C.
When I played the accordion in my church's orchestra as a teenager, I had to learn how to transpose music on sight. The pianist didn't like to play in keys that had more than one sharp. If a song in the hymnal happened to be in the key of D or E or A, she would hold up her hand and display the letter C, or else the number of flats we should actually play instead.
If the original song was in the key of D, we would all mentally transpose it down one step to the key of C and eliminate all of the sharps. If the original key were A or E natural, we would instead play
it in the key of A or E flat. I had to remember to use the new A flat or E flat as my tonic note and add the flats in my mind as we played. Apparently the pianist grew up taking piano lessons, and she always had difficulty with keys containing sharps, except for G, which only has one.