One of Philip Glass's most important early techniques was the additive process: beginning with a short musical pattern and gradually expanding it by adding notes—or shortening it by taking notes away.
Imagine a pattern of just three notes, repeated over and over. Now add another note, then perhaps two more. Each change is small, but gradually the pattern grows longer and takes on a different character.
Listen to an excerpt from Glass's Two Pages (1968), an early work built around the additive process. See if you can hear the repeating patterns gradually changing in length.
Unlike traditional musical development, in which melodies and harmonies are continually transformed, Glass could create an entire composition from the gradual expansion and contraction of a few simple musical ideas.
The notes themselves may hardly change—but the way we hear them certainly does!