Stand in front of a mirror and smile. If your front teeth are well proportioned, each one seems to step back a little from the one before it, in a steady rhythm. Dentists have been describing that rhythm with a number from art and architecture for about fifty years. It's called the golden proportion.

What the golden proportion is

It's a ratio of roughly 1.618 to 1, and it turns up all over the place, from the spiral of a shell to the layout of old buildings and paintings. In 1973 a dentist named Lombardi suggested that attractive front teeth follow it too, and in 1978 another dentist, Levin, turned the idea into a grid you could actually measure against.

It's about how wide each tooth looks from the front, not how wide it really is. Your teeth sit on a curve, so each one turns away from you a little more than the one before. In a "golden" smile, the visible width of the central incisor is 1.618, the lateral is 1 and the canine is 0.618. In other words, each tooth looks about 62 per cent as wide as the one in front of it. The photograph on our home page is marked up this way if you'd like to see it.

Where it helps

The ratio is most useful when we're rebuilding teeth from scratch, after an accident, with badly worn teeth, or when a full set of veneers or crowns is being made. It gives us and the technician a shared starting point for widths before a single tooth is shaped. It also explains why some smiles feel crowded or flat, for example when a lateral incisor is too wide or the canines look as broad as the front teeth.

Where it doesn't

That said, when researchers have measured large numbers of natural smiles that people rate as attractive, very few follow the golden proportion exactly. Plenty of lovely smiles have laterals a bit wider than the formula allows. Forced too strictly, the ratio can make the side teeth look skinny, especially on a broad face.

So we treat it as one guide among several. A front tooth usually looks right when its width is about 75 to 80 per cent of its length; much squarer and it looks stubby, much longer and it looks thin. Some dentists prefer a gentler, even step from tooth to tooth, sometimes called the recurring esthetic dental proportion, which can suit longer teeth better. The smile also has to match the width of the face and sit on its centre line.

And then there's what you want. Some people like a soft, youthful look and others a bolder one. If you like your own teeth, they're often the best guide we have.

How we use it

When we design a smile, we draw the proportions over photographs of your own face. If the golden ratio suits you, we use it. If it makes the smile look narrow or artificial, we adjust it. Either way you see the result on screen, and usually as a trial smile in your mouth, before anything is made.

In the next post we'll explain a few more words you'll hear at a consultation: lip line, smile arc and buccal corridors.