Go by the numbers when designing pot gardens. Whether you use a single pot, or group them in twos, threes, fours, fives or more, different sized groups function differently in the garden and produce distinctive effects.

One pot is the center of interest, like an only child. That very famous English garden, the White Garden at Sissinghurst Castle, is accented by one pot, a curved Provencal jar. The solid shape and dark color provide a focal point where paths cross and anchor the surrounding views of fluffy greenery.

A single planted pot at the end of a path will draw the eye down its length and complete the vista, while a sizeable potted specimen set where a path curves out of sight operates like a signpost saying "Come this way."

Center-of-attention pots can be distinctive and decorative and are best planted with a specimen worthy of the spotlight, whether it be a weeping Japanese maple, a spiky dracaena, or a sculptural succulent.

Two pots operate like gateposts, directing movement between them. Two pots announce an entry or doorway, flank walks, garden gates and steps. You can even use their gatepost-quality to signal a path at the far end of a lawn or orchard.

Paired pots should be identical twins: same pots same size, same plants. Paired pots are an ideal place to show off specimen plants since they're natural attention-getters.

Where something tall is needed, consider columnar junipers, conical topiaries and standards, plants trained or grafted into lollipop shapes. Nurseries carry many varieties in both flowering and foliage-only plants. If you choose citrus or rose standards, go for the smaller growing ones and set these thorny specimens back from where people will brush against them.

Three is a magic number in the world of aesthetics because three of an item is perceived as a unit by our pattern-making brains. When working with threes, don't frustrate this tendency towards unity. While each member of the trio needn't be identical to its mates, it should bear a family resemblance.

Use pots of similar materials, for example, all terra cotta, or all cast concrete or all glazed pots. Avoid big differences in sizes: group a 16- inch, a 12-inch and an 18-inch pot, for example, rather than a 6-inch pot with 24-inchers.

Go for correlations in plantings, too: a trio of low, mounding plants or a trio of grey-leaved plants, a trio of succulents.

Four pots are usually seen as to two pairs, but there are a few exceptions: Four identical pots planted with similar plants can be lined up to form a colonnade and are seen as a unit. This form is very useful as a divider or screen.

Four pots can become a complete garden picture when divided into one and three -- one large pot with three clustered at its feet. The big pot may or may not match the three: a Japanese maple in a large tub, ferns and begonias in glazed Asian stoneware.

Groups of five will often be divided into twos and threes by our mind's tendency to match up things, and larger numbers will be perceived as a group. But once you know the basics of pot math, you can combine and deploy pots to work for you.