Most cavities occur in the groves of back teeth.  This is where food is most likely to get stuck or with improper cleaning it can linger and cause decay.  These are sometimes called "brushing cavities." Another common area is on the smooth surface between teeth because this is another area that food can sometimes get stuck.  These can be called "flossing cavities."

I just saw a twelve year old boy who besides having about a 10 cavities, including 2  so bad that they were root canaled; had this unusual cavity on the cusp of his lower first premolar.

See the two pits on the front, or biting cusp?  When you see decay like this in a child you know that they have a very bad diet and it is more of what they eat rather than how much they brush that is the problem.  On questioning, we discovered that his diet consisted of low fat products and about 90% sugars and starches.  His home care was surprisingly excellent.

On opening the cavity, the decay had spread under the entire cusp and this is what the final preparation looked like.  Notice that the two pits were connected.  It is not unusual for decay to widen after it breaks thru the tooth enamel. Fortunately we have strong and esthetic filling materials now.  This is a picture of the final result.  It will provide years of service and with proper diet modification can last a lifetime.