Mandibular first premolar is the 4th tooth from the midline and second premolar the 5th tooth from the midline. This gives the two teeth different tooth numbers in various tooth numbering systems. Chronologically the first premolar erupts at the age of 10 to 12 years and second premolar at the age of 11 to 12 years. If we compare the measurements of the two teeth, the first premolar's crown is longer than the second premolar's crown, but the root is shorter, so that the total length of the two teeth remains the same. The mesio-distal diameter of the crown is same whereas, the bucco-lingual diameter is bigger in second premolar than first premolar. If we see the development, the first premolar develops from 4 developmental lobes but the second premolar may develop from 4 or 5 developmental lobes. This leads to the variations in the number of cusps, in second premolar, there are two cusps but in second premolar there are two cusps in two cusp-type and three cusps in three-cusp type. Moreover, if we see the number of functional cusps in first premolar, only the buccal cusp is functional as the lingual cusp is non-functional. Whereas, in second premolar all the cusps are functional with no non-functional cusps. Let us now study the morphological differences between the two teeth, here we will be studying differences between first premolar and second premolar of three cusp type only as it is the more common variety. Starting with the buccal aspect. The outline form of both premolars is trapezoidal. The buccal cusp of first premolar is sharper and longer than the buccal cusp of second premolar, which is short and less sharp. This is because in first premolar the cuspal ridges meet at a more acute angle than in the second premolar. The contact areas of both premolars are in the middle third however, the contact areas of second premolar are slightly more occlusal than in the first premolar. Moving to the root. In second premolar the root is thicker and has a blunt apex when compared to the root of first premolar. Next, the lingual aspect. From the lingual aspect all cusps are seen, in first premolar the buccal cusp and lingual cusp and in second premolar buccal cusp, mesio-lingual cusp and disto-lingual cusp. So we see, in second premolar there are two lingual cusps and in first premolar there is one lingual cusp, which is smaller and narrower. Now because of this in first premolar more of occlusal surface is visible as compared to the second premolar. Additionally in first premolar there is significant lingual convergence, because of which more of mesial and distal surfaces can be seen. Whereas, in second premolar, lingual convergence is minimal because of which lesser mesial and distal surfaces are seen. In first premolar a mesio-lingual developmental groove runs adjacent to the lingual cusp. Whereas, in second premolar a lingual developmental group runs between the two lingual cusps. Next, mesial aspect. Both premolars have a rhomboid outline form. In first premolar both buccal cusp and lingual cusp are visible, whereas in second premolar the buccal cusp and only the meso-lingual cusp are visible. In first premolar the lingual cusp is just two third the height of the buccal cusp whereas, in second premolar the mesio-lingual cusp is smaller than the buccal cusp, but not that small. In first premolar the mesial marginal ridge is slanting whereas, in second premolar it is horizontal. In first premolar the mesio-lingual groove is visible from this aspect, but in second premolar there is no such groove present. Now, coming to the distal aspect. In first premolar like the mesial aspect, both the buccal cusp and lingual cusp are seen but in second premolar all three cusps, the buccal cusp, mesiolingual cusp and disto-lingual cusp are seen. Interestingly, in first premolar more of occlusal surface is visible from the mesial aspect than its distal aspect, on the other hand in second premolar, more of occlusal aspect is visible from the distal aspect than mesial aspect. Finally, the occlusal aspect. The outline form of the crown of first premolar is diamond shaped and that of second premolar is square shaped. In first premolar both cusps are seen whereas in second premolar all three cusps are seen. In second premolar two triangular fossa, distal triangular fossa and mesial triangular force are present. Whereas in first premolar the mesial fossa and distal fossa are not triangular in shape but linear and circular, hence not called triangular fossa. The second premolar has a central pit in the middle of the occlusal surface, but in first premolar no such pit is present. Lastly, both premolars have 3 developmental grooves, mesial developmental groove, distal developmental groove but the third groove in first premolar is mesio-lingual developmental groove whereas in second premolar it is lingual developmental groove. Moreover, the grooves in second premolar form a Y-shaped pattern not present in first premolar.