Good design is knowing where to draw the line.
I spent the best part of the nineties and first decade of the naughty's designing gardens pretty much non-stop. Besides all the to-ing and fro-ing between wish lists and budgets, the design could usually be broken down into a set of simple questions. The key one was "So, how do you like it, straight or curvy?" - which I admit, as I write it, seems more like a toe in the pool in an internet chat room rather than a serious question exploring the virtues of alternate aesthetic variations in the garden design process. But, importantly, the answer helped me pursue the most appropriate design line for a particular garden. The interesting part was how to express that line.
In a typical lawn, for example, I almost always started by drawing the lawn before I drew the garden, as the lawn is the dominant element. I also almost always tried to frame the lawn using a mowing strip. This is a hard surface which is flush with the lawn, giving the lawn mower unimpeded access to the edges of the lawn. It's also a great opportunity implement line. The materials you use can change the way you see the garden.
Let's take a square lawn as an example. If we use old brick, side by side and point in between each one with mortar, we send a rather different message than if we use terrazzo (polished concrete) with a coloured glass inlay. Both materials have a kind of currency, both contribute to a design language and depending on the one you choose, the story in the garden will be different. A structure as simple as a mowing strip can set an atmosphere which echoes throughout the rest of the garden design.
Planting can add strong definition in conjunction with other elements such as mowing strips or paths but also function as an edge on its own. Hedging is a classic example. Box hedges in particular have been used for hundreds of years as an expression of line in a garden setting. There's a tendency when using hedges to just run them around the edges like a kind of plant wall. Fair enough, this kind of garden can be charming but it can also be unnecessarily obvious. A bit more interplay with other aspects of the garden can make for a lot more interest. By setting parts of the hedge back from the edges or shaping the hedge differently from the edges, then planting bulbs, annuals or colourful edibles in the resulting gaps, you successfully dissolve the monotony.
Large drifts of smaller plants and ground covers can also define garden edges. The most popular of these plants seems to be mondo grass (ophiopogon japonica). This is primarily because of its reliability and tough nature but annuals such as celosia, lobelia, marigolds and a myriad others are definitely worth the effort. For winter colour you can't go past cyclamen. Garden design, as I mentioned earlier, has a language of its own which has developed over hundreds of years and I think interesting garden design is about being clever with what you've got rather than reinventing the wheel. Or the straight line.