FIGURE F16.2.1 • Classification of permanent and deciduous teeth. Three systems of tooth classification are used. The central panel of the diagram shows the permanent teeth, whereas the upper and lower panels show the deciduous teeth. Dentition is divided into four quadrants: upper left (UL), upper right (UR), lower left (LL), and lower right (LR). Each quadrant includes 8 permanent teeth or 5 deciduous teeth. In the American (Universal) system (blue), permanent teeth are designated with Arabic numerals. The numbering begins from the wisdom tooth in the upper right quadrant designated as tooth number 1 and continues along all the teeth in the maxilla to tooth number 16, which designates the third upper left molar. The numbering progresses to the mandible, beginning at the third left lower molar designated as number 17 to the third lower right molar designated as number 32. In the American system, deciduous teeth are marked with capital letters designated for each tooth. The pattern is the same as that for permanent teeth, so the numbering begins from the second upper right molar and finishes with the second lower right molar. In the International system (red), also referred to as the Two-Digit System, each tooth is designated with two numbers: The first number indicates the dentition quadrant, which is marked from 1 to 4 and from 5 to 8 in clockwise direction beginning from the upper right quadrant for permanent and deciduous teeth, respectively. The second number specifies individual teeth in each quadrant beginning from the midline where the medial incisors are designated as number 1 and third molars are designated as number 8. In the Palmer system (yellow), the dentition is divided into four quadrants with a right-angle bracket. The vertical line of the bracket divides the dentition into a right and a left side beginning at the midline. The horizontal line of the bracket divides the dentition into the upper and lower parts to designate teeth in the maxilla and mandible. In the Palmer system, permanent teeth are numbered with Arabic numerals beginning from the midline. The deciduous teeth are marked with capital letters also starting from the midline. To mark a particular tooth with the Palmer system, two lines (vertical and horizontal) and the correct number or letter of the tooth are needed. (Table design courtesy of Dr. Wade T. Schultz.)
Teeth consist of several layers of specialized tissues.
Teeth are made up of three specialized tissues:  Enamel, a hard, thin, translucent layer of acellular mineralized tissue that covers the crown of the tooth.
 Dentin, the most abundant dental tissue; it lies deep to the enamel in the crown and cementum in the root. Its unique tubular structure and biochemical composition support the more rigid enamel and cementum overlying the surface of the tooth.
 Cementum, a thin, pale-yellowish layer of bone like calcified tissue covering the dentin of the root of the teeth. Cementum is softer and more permeable than dentin and is easily removed by abrasion when the root surface is exposed to the oral environment.
Enamel is the hardest substance in the body; it consists of 96 to 98% calcium hydroxyapatite.
Enamel is an acellular mineralized tissue that covers the crown of the tooth. Once formed it cannot be replaced.
lines of Retzius