Momentum Shell

Energy shell of a harmonic oscillator in phase space. Every microstate of fixed energy $E$ lies on the ellipse, whose semi-axes $\sqrt{2E/m}$ and $\sqrt{2E/k}$ are set by the mass and the spring constant. Counting the states in a thin shell around this curve is what defines the microcanonical entropy. See also ergodic, which adds a trajectory filling the shell.