Two pairs of premolars are present in both maxilla and mandible, 1st premolar and 2nd premolar. Let us see the differences between the premolars of the two arches, we'll start with some general features. Premolars are also known as bicuspids, as they have 2 cusps. However, the term bicuspid is not ideal for mandibular premolars, as 1st premolar has only one functional cusp and the 2nd premolar, more commonly, has 3 cusps. In this video we will be considering only the 3-cusp type of mandibular 2nd premolar. In maxillary premolars the crown shape is similar to each other, however in mandibular premolars they are very different. But if we consider the roots, both mandibular premolars have same number of roots, whereas in maxilla they are different, as 1st premolar has 2 roots. Both maxillary premolars develop from 4 developmental lobes, however in mandible only the 1st premolar develops from 4 lobes, as 2nd premolar develops from 5 developmental lobes. Coming to measurements. Both maxillary and mandibular premolars are almost of equal dimensions from all aspects, except buccolingually, where the maxillary premolars are bigger than the mandibular premolars. Therefore in general, it can be said that maxillary premolars are larger than mandibular premolars. Also within the arch, if we look at their dimensions, both maxillary premolars appear equal in size. But if we consider the two roots, first premolar is larger than the 2nd premolar. However in mandible, 2nd premolar has greater dimension than the first premolar, additionally, the 3 cusps make the 2nd premolar the larger of the 2 premolars. Now let us study the morphological differences from Individual aspects, starting with the buccal aspect. In both mandibular premolars, to the midline axis of the tooth, the buccal cusp tip lies to the mesial side. However only in maxillary second premolar, the buccal cusp tip is mesial, as in 1st premolar it is distal to the midline axis. Similarly in both mandibular premolars the disto-buccal cuspal slope is longer than the mesio-buccal cuspal slope. However in maxilla, only in 2nd premolar the disto-buccal cuspal slope is longer, as in 1st premolar the mesio-buccal cuspal slope is longer than the disto-buccal cuspal slope. Next, lingual aspect. In mandibular premolars the lingual cusps are smaller in size in comparison to the lingual cusps in maxillary premolars. Moreover in mandibular premolars the difference between the size of the buccal cusp and lingual cusp is more as compared to maxillary premolars, where they are almost of same size. Due to this difference in mandibular premolars, more of occlusal surface is seen, than seen in the maxillary premolars. In both mandibular premolars, a developmental groove is seen. In mandibular 1st premolar it is the mesio-lingual groove, whereas in 2nd premolar it is the lingual groove. No such groove is there in maxillary premolars. Mesial aspect. The outline form in maxillary premolars is trapezoidal in shape. Whereas in Mandibular premolars it is rhomboidal in shape. This is because, on the central axis of the tooth the crown of both the mandibular premolars has a lingual tilt. However it is not so in maxillary premolars. In maxillary premolars, both the cusps are within the confines of the root, whereas in mandibular premolars the lingual cusp is outside the lingual outline of the root. Moreover with respect to the midline axis, mandibular premolars have buccal cusps nearer to the midline axis than maxillary premolars. Mesial marginal ridge in both maxillary premolars is horizontal, whereas in mandible it is horizontal only in 2nd premolar, but in 1st premolar it is oblique. Next, distal aspect. The differences seen from the mesial aspect are same in the the distal aspect as well. Finally, the occlusal aspect. In maxilla the outline form of 1st premolar is hexagonal, in 2nd premolar it is oval. Whereas in mandible, in 1st premolar it is diamond shape and in 2nd premolar it is squarish. In maxillary premolars the buccal cusp and lingual cusp are at equal distance from the center of the tooth. Whereas in mandibular premolars, the buccal cusp is nearer to the midline than the lingual cusp. In both maxillary premolars, mesial and distal triangular fossa are present, but in mandible only the 2nd premolar has 2 triangular fossa, as 1st premolar has mesial fossa and distal fossa, which are not triangular, but are of linear shape and circular shape. In both maxillary premolars, a central developmental groove is seen running mesiodistally between the two cusps. Whereas in both mandibular premolars, no central developmental groove is present.