Everybody knows when someone's teeth look great, but sometimes it is difficult to work out just what it is about them that makes them look so good. This can particularly be the case for somebody considering cosmetic dentistry or orthodontics. Whilst knowing they are unhappy with the way that their teeth look it can be difficult to work out what it is that needs to be changed.

Often patients are not sure whether it is the shape of their teeth that are the problem or if it is their alignment. Sometimes it may be a combination of the two. It can be very difficult for patients to visualise what they might look like with a straighter more uniform smile and whiter, more even teeth.

Cosmetic dentists and orthodontists have several principles that help them to design the perfect smile.

The width of your two front teeth, divided by the length of those teeth should ideally be 77%. The closer they are to this proportion the better they will look. Often it is hard to judge this for yourself, however it can be surprising when after orthodontics you notice how uneven your teeth look. This was the case with my own teeth. This can be solved either by having some reshaping where the dentist files your teeth, by using bonding which builds up the tooth shape, or veneers which are placed over existing tooth structure.

As far as possible the smile line of your teeth should match the line of your lower lip. This helps to give a more youthful appearance.

In a perfect smile we can see the progression from front to back teeth and the smile appears wide. In a narrower smile the back teeth can appear to be in shadow. This can make the front teeth look as if they are alone. For a perfect smile all teeth should be visible, although this may mean updating non white fillings in molars. This is often what people mean when they talk about widening their arch. It can be achieved by veneers to make the teeth fatter or by orthodontics to move the teeth into a new position. 

In a perfect smile the front teeth should appear to be in the very centre of the mouth with the line between the two front teeth (the midline) central. This midline should also match up with the midline of the bottom teeth. In reality this can be difficult to achieve and many people look like they have very good teeth without achieving this.

Embrasures are the tiny gaps at the bottom of the teeth as shown in this picture. They should be small at the front teeth and become larger as they move towards the molars. Younger people generally have larger embrasures. Embrasures are one of the things that stop teeth looking like false teeth or piano keys. Teeth without them often have the appearance of false teeth. You should query your dentist about this as some cosmetic dentists do not take this into account and can produce very false looking teeth. 

These are just some of the principles that a cosmetic dentist might use whilst evaluating your smile. One thing they may not tell you however, is whether your smile is already perfect for you. Whilst perfect teeth can look great, sometimes it is the tiny imperfections that give a face character. Kate Moss has small snaggle teeth at the front, but these actually help to make her face more memorable.

Ask a parent or trusted friend- one that you know will be honest- whether you smile needs improving before you embark on a costly and time consuming process that might just change what makes you, you.