If a 4th root canal is present in a maxillary 1st molar, it will likely be located in which of the following roots?

1) Mesiolingual

2) Mesiobuccal

3) Distolingual

4) Distofacial

Well over 50% of maxillary 1st molars have two canals in the mesiobuccal root. Some authors cite the number as high as 90%.

There are usually 3 roots and 4 canals in a maxillary 1st molar.
The fourth canal, the mesiolingual canal, is also referred to as: the accessory mesiobuccal canal, mesiocentric, mesiopalatal canal or MB2. 
It is usually tough to find MB2 through clinical inspection and it is not obvious on a radiograph. A dentist should expect it is present before beginning a root canal on the first maxillary molar.

Which of the following is true regarding maxillary molars?
1. Curve inward more sharply from the medial plane on the buccal than do the mandibular molars
2. By their arrangement and design allow room for the powerful masseter muscle.
3. Are set somewhat lingual to the premolars
4. Show a convergence of their buccal surfaces mesially