How to understand why apple trees need to be pruned

Each year an apple tree should produce three things: new growth, fruit buds on last year's and older growth, and fruit on those fruit buds formed in previous years. In order to keep an apple tree in balance and fruiting, one must prune. However, pruning is often done poorly. In this, the first installment of his series on practical apple tree pruning, Stephen Hayes of Fruitwise Heritage Apples looks at the general structure of the apple tree.