The art of espalier―training plants to grow in a flat plane―is a time-honored way to reap a generous harvest of fruit from a small space. Espalier styles vary. Half a dozen designs are shown above; most kinds of fruit trees can be trained to one or another of these.
Citrus trees, however, produce more fruit if grown as an informal espalier―that is, one without a precise pattern. You plant the tree in front of a structure and allow it to branch naturally, then remove any growth that juts out too far.
Step-by-step: How to make a wire espalier