The CLEAVE option allows the user to replace an interior vertex with two new vertices, each of which is connected to roughly half of the neighbours of the vertex being replaced. To carry out a cleaving operation, it is necessary only to enter C and use the cursor to select the point to be replaced. The purpose of this option is to allow convenient insertion of extra vertices. Cleaving is not allowed on boundary vertices or with interior vertices having only three or four neighbours. Examples of cleaving are shown in Figure 5.5. The depth at each new vertex position is computed automatically by linear interpolation, but it is recommended that, for better accuracy, all depths should be recomputed using program REDEP at the end of the editing session if CLEAVE has been used. The RESHAPE option below is often used following cleave operations, in order to improve triangle shape.