The skill in achieving exactly the right effect lies in the choice of materials and the placing of the structures. For example, an arch placed at random on a lawn or across a flower bed is unsettling, but one that straddles a path makes more sense to the brain, so pleases the eye.

Trompe l'oeil

You can even place an arch on your boundaries if you take a path to it, and disguise the lack of an exit with a strategically-placed mirror. Such illusory devices are known as trompe l'oeil (ie a trick of the eye) and are fun to try out. It's all about altering perspectives by tweaking components, especially of trellis, to convince the eye that there is more depth to the scene than there is in fact. This relies on straight lines being angled instead of parallel to each other, and sometimes the inclusion of a mirror or two. Mirrors, slightly angled to reflect plants rather than you when viewed, add greatly to the illusion, but make sure they are sealed for outdoor durability.

Width is important

Whether to make an illusion convincing, or for practical reasons, an arch should be wide enough for a person to pass through without snagging on the plants that it supports. If you want romantic roses, then go for as wide an arch as possible to give yourself clearance; that applies to arbours too. After all, airy bowers are far more appealing than claustrophobic corners.