An espaliered pear tree.
Espalier is an art that's seeing a revival as more people find reason to grow their own fruit and discover that growing a flat tree is not difficult to do and requires no more time than growing a tree "in the round." Both must be pruned to maintain shape and production; an espalier is simply pruned differently!
We worked with a client this year to train a young peach tree espalier, and reached for our files meaning to leave him with diagrams from a What's Up issue... only to discover that our diagrams are in our Garden Walls class and talks. So there's another bit of our library gone onto our list to be posted as soon as we can.
If you like our way of explaining things and would Sponsor us, please just name your topic. We will use the time your Sponsorship "buys" us to dip into our library, convert what we find there, and share it here.
Meanwhile, refer to the book American Horticultural Society Pruning and Training (pages 114-115), or search for articles by practitioners of the art, such as written by rosarians at the New York Botanical Garden or horticulturists at the Chicago Botanic Garden's home garden center.