Maxillary premolar: The point of entry in the coronal direction is located in the center of the symphyseal surface, following the direction of the tooth’s long axis. Because the pulp cavity is relatively wide in the labiolingual direction, the pulp cavity should expand in the labiolingual direction with a narrow intermediate proximal and distal mesial diameter. This tooth generally has one or two root canals (Figure 5-8). Maxillary molar: Generally the two buccal root canal openings and the palatal root canal of the maxillary molar form a triangle, and it is not very difficult to find one or two root canals among many calcified pulp cavities and root canals, however, the remaining root canal openings may not be well explored, in which case a subjective awareness of the planar geometry of the root canal openings is necessary (Fig. A). In the case of maxillary molars, these teeth have a high incidence of four root canals (Figure B). We can usually find a second root canal (MB2) in the proximal buccal root, or a notch shape located on the palatal side of the proximal mesiobuccal root orifice. the MB2 can be anywhere from 0.5-5.0 mm from the palatal root canal if present. As shown in Figure A: A synoptic view of a maxillary molar shows the location of a standard root canal orifice, and by locating two root canals, the positioning of the third root canal (X) is more fixed.5-9B: After the three root canal orifices have been located, MB2 may be positioned approximately 0.5-5.0 mm from the palatal root canal orifice, with most MB2 located approximately 0.5-5.0 mm from the original proximal 1-3 mm of the buccal root canal opening. Mandibular incisors, cuspids and premolars: Normally the root canal morphology of these teeth is single canal; however, currently we can often find a second root canal present on the lingual side of the root canal. In incisors and cuspids, the second root canal is rarely explored due to the angle of the standard cavity-shaped entrance of the anatomical crown or lingual surface, or the presence of calcification. After probing and shaping the primary root canal, it is important to continue extending the lingual surface of the root canal opening and probing the second root canal with a 10# or 15# K file, and if no root canal is explored, the lingual side can be gently swept with a rounded GG 2, 3 and 4 drill or root canal former This helps to find the lingual root canal opening. There is also a risk of the instrument breaking during the probing process, however, most of them break at the shank, so even if they do break, they are easier to remove from the root canal. Figure A: In a mandibular incisor with a double root canal, the normal root canal opening is first entered through the labial root canal, followed by probing the lingual second root canal; Figure B: In mandibular cuspids and premolars, the second root canal is almost always located lingually, and most branches form an angle away from the labial root canal, requiring more delicate instrumentation to probe; Figure C: Two separate adjacent root canals of an isolated mandibular premolar. Mandibular molar: The morphology of the root canal opening of most mandibular molars is a triangle consisting of two root canal openings in the proximal mesial root and an oval distal mesial root canal opening. The distal mesial root canals are usually wider labiolingually and morphologically require an independent labiolingually oriented root canal preparation; approximately 30% – 50% of this root canal is independent and must be fully explored in the calcified distal mesial root canals. It is also uncommon for both roots of the mandibular second molar to be single canals. If one root canal opening is found in the proximal mesial root, it is still necessary to probe the other root canal opening as described above. In a mandibular molar with two canal openings, the proximal and distal mesial root canals are usually located on the proximal and distal mesial line (Figure A). If, when probing the proximal mesial root canal, it is found that the proximal mesial root canal is not on this line relative to the distal mesial root canal, but is located in the proximal or distal mesial part of this line, then the proximal mesial root canal may have two root canals and we can imagine the possible location of the second root canal opening based on the formation of three or four canal openings in the mandibular molar ( Figure B). This method can also be used to investigate calcified root canal openings, which are very close to the true root canal opening.