MODELLING CONCEPTS
Ridge clusters.
A ridge cluster is a set of six vertices which begin a change of direction (normally inside a boundary). The angles between some of the vertices which are naturally created during extrusion with scaling along a plane, create an arrangement of angles which are not well interpreted by the subdivision surface modifier and need to be fixed.
A corner vertex and the two vertices to either side of it form the top of a ridge cluster and these are normally left in place. The three vertices that these are directly connected to in the direction of a boundary edge. They are normally fixed before control loops for that boundary are inserted.
A Ridge Cluster
A fixed Ridge Cluster
The two vertices at the outside are normally placed incorrectly during an extrusion and need to be moved until they are perpendicular to the top vertices in the cluster.
You can see a (probably much clearer!) demonstration of this exact process when we create The King for the chess set.
If they are not fixed, a thin ridge will be apparent in renders and the general spacing of your vertices will not be optimal for deformation and general use.
Ridge clusters can also be used to your benefit in recreating shapes and ridges often seen in architectural design!