Changing written sheet music from key to key involves a process called transposition. For music, transposition is a term denoting the musical calculation necessary to change a musical composition from one key to another. Transposing music by hand, in sheet music, is a somewhat laborious task involving manually rewriting the musical score note by note. It also involves the exercise of musical judgment as to how to indicate notes and how to best keep the transposed music easy to read.
Determining the purpose of the change in key from the original
Determine the musical key(s) of the original sheet music. Analyze the original composition by looking at its key signature(s). Analyze the most important keys used in the structure of the entire composition.
Determine the purpose of the transposition. Consider the type, genre, and complexity of the original. Decide why the transposition is desirable. For example, a song may be transposed to better fit a particular singer's voice range.
Choose the key that best suits the purpose of the transposition. Consider the ranges of the instruments and voices involved in the original. Make certain to choose a key that is appropriate for the instruments and voices that will perform the transposed version.
How to transpose the original to the new key
Music transposition requires determining the interval, the distance between notes, of the original key(s) and the new key. For example, if the basic key of the original is G major and you want to change the key to D major, you have to change all the notes either five whole steps upward or four whole steps downward. The distance between G and the D above it is five whole steps, e.g., G, A, B, C, D, also called a perfect fifth. If transposing to this higher D is impractical, or makes the sound too high a change from the original, you may transpose downward four whole steps, e.g., G, F#, E, D, also called a perfect fourth.
Exercise musical judgment in determining how to spell the transposed notes. Notes in music may be represented or spelled enharmonically. Enharmonic means the same pitch may be spelled two different ways. The note G sharp may also be spelled A flat. If you transpose the original from G to D, and the original contains a G sharp, it makes sense to spell it D sharp, transposing five steps upward, because both G and D are keys containing sharps. But if instead you transpose the original from G to A flat, you might spell the transposed G sharp as E flat, enharmonically the same note, because A flat contains flats and the performer would read the E flat more easily than a D sharp.
If you'd like to avoid manual transposition, choose a music program for either your Mac or PC that will write notes out in the format you specify. Many such programs now exist with varying amounts of notation options.
Scan the original sheet music into your computer. Or if you have downloaded the sheet music, you may use that file. Input the sheet music file into the notation program, usually by selecting the file and adding it to your notation programs files or dragging and dropping the file. Open the file in the notation program and choose the transpose option from the indicated menu. The computer will do the mathematical mechanics of the transposition for you. You will then still have to choose note spellings and choose whether the transposed version needs to be moved up or down an octave, or eight note perfect interval, in order to sound in the desired range.
Print out the sheet music from the transposition in order for the performer(s) to read it. Many programs allow you to choose to score your music for differently sized performing groups and instruments or voices. The program capable of doing the transposition will probably also play back both versions so you may hear how successful your transposed version is.
Copyright may restrict your ability to use or perform your transposition of the original. Unless your use is just for yourself privately, check the copyright law that applies.