Transposing instrument
Having transposing instruments makes it possible for pipes to play in several different keys.  The fingering can remain the same. Only the key the music is written in, the notes that harmonize with that key, and timbre (what the note sounds like, not just which key it is) change as the different sizes of instrument are used.  A cor anglais is like an oboe but a fifth lower (Five notes of a scale. A written C sounds like an F).  Any oboe player can play the cor anglais, reading the music and playing with normal oboe fingering.  It will automatically sound a fifth lower.  Compare this to recorders which are not transposing instruments.  On a descant recorder, the note played with three fingers of the left hand (1-2-3-0-0-0) is a G.  To play a G on a treble (alto) recorder the fingering is 1-2-3-1-2-3.  This can be confusing at first, but with practice players can get used to changing between the different sizes of recorder.