The Android pattern lock screen consists of 9 contact points arranged in a 3 x 3 matrix. Acceptable patterns are conditioned by three main rules...
- A minimum of four contact points must be connected, and the maximum number is obviously 9.
- A contact point can only be used once!
- An intermediate point between two connected points, must be included in the pattern order, unless it was previously used!
(vía: CHM Tech)