Order the phases of one full cycle of a mass on a spring (starting from maximum compression).

1. Maximum compression ($x = -A$)
2. Passes through equilibrium moving right ($x = 0)$
3. Maximum extension (x = +A)
4. Passes through equilibrium moving left ($x = 0)$
5. Returns to maximum compression ($x = -A$)

SHM is periodic: compression $\to$ equilibrium $\to$ extension $\to$ equilibrium $\to$ compression.