In a general statement the ridge is any horizontal framing member that either a common, hip or valley rafter can be nailed to. In earlier times almost all roofs had no ridge. The opposing commons where just nailed to each other. Over time the use of a 1x was used and it proved to aid in the "nailing" of the commons. Nowadays, just about any size ridge can be used. One thing to bear in mind is that the "larger" the ridge is the greater the load the roof must carry.

The one special type of ridge is what is known as a "supported" ridge. This means that the ridge acts as a "beam" and carries some of the roof load. The ridge can only be supporting if it is supported down to the foundation.