In most dental offices, after the hygienist cleans your teeth, the dentist comes in to examine your teeth. Then, out of nowhere, he or she starts rattling off alpha-numeric jargon like, 3MOD, 5DO, 13MFD and so on. The dentist is not looking at a bowl of alphabet soup, but, rather, using a form of dental shorthand. The numbers represent which teeth have cavities or other problems. Tooth number one is the upper-right third molar or wisdom tooth, the farthest tooth back in the mouth. Tooth number 16 is the upper-left third molar. Tooth number 17 is the lower-left third molar, and tooth number 32 is the lower-right third molar. So, teeth eight and nine are the upper-front teeth, or the left and right central incisors, and teeth 24 and 25 are the lower-front teeth, or lower-left and right central incisors.
Source: http://www.dentistry.com/