Dynamics, articulation and form
Crescendo and diminuendo
Crescendo means getting gradually louder; diminuendo means getting gradually softer. They are dynamic signs that show not a level of loudness but a change in it.
Dynamics can change not only in a jump from p to f, but little by little. A crescendo is written as an opening wedge under the notes, narrow at the start and wide at the end: the sound grows, each note a little louder than the one before. A diminuendo is a closing wedge: the sound fades.
Instead of the wedges, often called hairpins, you may see the shortened words cresc. and dim. (or decresc., decrescendo, which means the same as diminuendo). They mean the same thing; the wedge simply shows more exactly where the change begins and ends.
A wedge usually leads from one dynamic sign to the next, for example from p to f. On the piano a crescendo is played by pressing the keys with gradually more weight — evenly, without jumps.
What it sounds like
A crescendo sounds like a wave coming in, a diminuendo like one going out. It gives a melody direction: it goes somewhere and comes back.
Where it is taught: the studio’s interactive textbook, chapter 17, “Dynamics and articulation”.