Sounds, including music, can be barely audible, or loud enough to hurt your ears, or anywhere in between. When they want to talk about the loudness of a sound, scientists and engineers talk about amplitude. Musicians talk about dynamics. The amplitude of a sound is a particular number, usually measured in decibels, but dynamics are relative; an orchestra playing fortissimo is going to be much louder than a single violin playing fortissimo. The exact interpretation of each dynamic marking in a piece of music depends on:

Traditionally, dynamic markings are based on Italian words, although there is nothing wrong with simply writing things like "quietly" or "louder" in the music. Forte means loud and piano means soft. The instrument commonly called the "piano" by the way, was originally called a "pianoforte" because it could play dynamics, unlike earlier popular keyboard instruments like the harpsichord.

When a composer writes a forte into a part, followed by a piano, the intent is for the music to be quite loud, and then suddenly quite soft. If the composer wants the change from one dynamic level to another to be gradual, different markings are added. A crescendo (pronounced "cresh-EN-doe") means "gradually get louder"; a decrescendo or diminuendo means "gradually get softer".

A composer may want a particular note to be louder than all the rest, or may want the very beginning of a note to be loudest. Accents are markings that are used to indicate these especially-strong-sounding notes. There are a few different types of written accents (see Figure 3), but, like dynamics, the proper way to perform a given accent also depends on the instrument playing it, as well as the style and period of the music. Some accents may even be played by making the note longer or shorter than the other notes, in addition to, or even instead of being, louder. (See articulation for more about accents.)