Instruments such as a trumpet and clarinet (tuned to Bb) and alto saxophone (tuned to Eb) require transposing. For example, when the written note C is played on a trumpet, it is the same note as an Bb on a piano. Other instruments requiring transposing include English horn, alto flute, baritone sax, and French horn.
For the pianist, it means they must be aware of the keys of these different instruments. For example, if the pianist is accompanying a trumpet player, they need to realize that communicating with them will be difficult unless they “transpose” notes when they speak of them. If they say to the trumpet player, “Let’s start at measure 14, where you come in on the note C,” the pianist is, in fact, looking at the note Bb on their piano music. This ability takes time to learn. But in the case of a piano and a trumpet, the pianist knows that every note they see on their music is a whole note above what the trumpet is playing. The same care has to be taken when purchasing music. Most sheet music of this type will say something to the effect of, “written for piano and Bb trumpet.”
Seasoned musicians know how to do this and after a few months of training (and performing with a number of different instruments), transposing becomes second nature.
When it comes to a vocalist, there is of course, no transposing needed. For example, a vocalist in the jazz group above, will speak (in terms of transposing and instruments of different keys) the same way as the pianist. The vocalist sings the notes C, D or G in the same way, and on the same pitch, as they are played on the piano. Instruments such as guitar, mandolin, banjo, and recorder do not require transposing.
If the pianist is a composer, these differences pose quite a challenge when composing for a number of different instruments. If the composing is done on paper, large “score” paper must be used to easily see the different parts for each instrument. And every time the pianist writes a part for a trumpet, sax, or other instrument that requires transposing, they have to be adept in writing out the part in a different key, paying attention to all the rules and requirements for that key (such as sharps and flats).
Although learning the different instruments in different keys can be difficult at first, it makes for a very well-rounded pianist and musician.