Late winter, just as the sap begins to rise, is the time for grafting. Apples grown from pips do not grow true to their parent (scion), so different varieties are grafted - a process in which the wood of the parent tree is bound to a suitable rootstock. The fact that it works continues to amaze us, thousands of grafts later.
We do various types of grafting. Rind grafts, in which scion wood is inserted under the bark of a mature tree. Saddle grafts (as in the pictures), in which scion wood is spliced onto the wood. Cleft grafts, where the scion wood is inserted into a cleft split into medium sized wood.