Human dentition consists of two sets of teeth: primary or deciduous or milk teeth that are 20 in number and permanent teeth that are 32 in total. These teeth are incisors, canines and molars. Since these teeth have two life cycles these are known as diphyodont teeth. However permanent dentition also shows presence of premolars and third molars that are not seen in the primary dentition. Hence these teeth are said to have only one life cycle and are known as monophyodont teeth. These are three per quadrant (two premolars and one-third molar). Hence a total number of monophyodont teeth is $3\times 4=12$.