When we look at a smile, we're really looking at four things stacked on top of each other: the teeth, the gums that hold them, the lips that frame them and the face around all of it. We go through them one at a time.
The teeth
Six upper teeth do most of the work in a smile. In the middle are the two central incisors, the biggest ones, which set the tone. Either side of them are the lateral incisors, a little narrower and slightly shorter, which soften things. Then come the canines, the pointed ones that turn the corner. How much of the canines you show has a lot to do with whether a smile looks narrow or broad.
Each tooth also has its own shape, more square, oval or tapering, and its own surface texture and edges. Young teeth tend to have rounded corners with tiny gaps between them at the edges. With years of chewing those edges flatten out.
The gums
Think of the gums as the mount around a framed picture. Healthy gums are firm and pale pink, and they rise into a little point between each pair of teeth. The highest point of the gum over each tooth is called the zenith. On the front teeth and canines it sits slightly towards the back of the tooth, and on the laterals it's usually a touch lower. When those heights are uneven, even perfect teeth can look crooked.
The lips
Your lips decide how much of all this anyone actually sees. With your lips relaxed and slightly apart, a few millimetres of your top front teeth usually show. That amount shrinks as we get older, and it's one reason an older smile can look tired. When you smile fully, your top lip rises. If it lifts well above the gum line, you'll show more gum than most people are comfortable with.
The face
Finally, the smile has to sit well in your face. Its centre should line up with the middle of your face, its curve should follow your lower lip, and its width should suit the width of your face. It should also suit you. A very square, very white smile can look striking on one person and false on another.
Why the order matters
We plan from the face inwards but treat from the inside out. Health comes first, then the teeth, then the gums, and then the lips and face if they need anything at all. Every treatment later in this series works on one or more of these layers. Whitening and veneers change the teeth, laser contouring changes the gums, and fillers and anti-wrinkle injections change the lips and face. A good plan decides which layers need attention, and in what order, before starting on any of them.
Next time: the golden proportion, and why the widths of your front teeth often follow a fairly neat pattern.