Teeth in each of human jaw (upper and lower) are arranged in a semi-circular shape. In dentistry, this layout of teeth in one jaw is called an arch. Upper teeth form the maxillary arch while the lower teeth form the mandibular arch. Half of an arch or a hemi-arch is called a quadrant. In a primary dentition each arch contains ten teeth: two central incisors, two lateral incisors, two canines, two first molars and two second molars, making a total of twenty teeth in a permanent dentition. Permanent dentition consists in each arch of six molars, four premolars, two canines, and four incisors making a total of thirty two teeth in the two arches.
Maxillary (upper) arch is normally slightly larger than the mandibular arch. While the two arches are more or less aligned at the back ends, when the mouth is closed the maxillary dental arch comes over and in front of the mandibular arch. The central incisors in the upper arch are wider than those in the lower arch and the molars in the lower arch are wider than the molars in the upper arch.
Normally the teeth in the lower arch grow earlier than the corresponding ones in the upper arch. Length of the arch is very important. If the arch is two short, teeth will have to grow closer and may be overlapped, tilted, or incorrectly placed, resulting in overcrowding as well as malalignment. This can create problems in chewing and speaking, and may lead to teeth cavities, gum and periodontal infections. Correction of tooth misalignment can be done with the help of orthodontic appliances, such as teeth braces or Invisalign removable clear aligners. In some cases, where there is not enough room for all the teeth, one or two teeth may need to be extracted on both sides.