Upper second molars – #15 and #2
The big back teeth between the first molars and the wisdom teeth in the upper jaw.
Roots
Usually three (92% in a Saudi scan study). In a sizeable minority the roots are fused together: anywhere from 9% to 44% across studies.
Canals
Usually three. A fourth canal (MB2) is less common than in the first molar: about 23% in a pooled analysis of Han Chinese patients, and 12% to 28% in a Saudi study, depending on how far down the root you look.
These are typical counts from scan studies. Teeth vary, and only an X-ray or scan shows what a particular tooth has.
When they come in
Usually at 12–13 years old, according to the ADA's eruption chart.
No baby teeth come before them: adult molars come in behind the baby teeth.
They bite against the lower second molars.
Sources
Alamri et al. 2020; Ghasemi et al. 2017; Jo et al. 2016; Martins et al. 2024; Wu et al. 2026; American Dental Association 2012; Harris 2005. Full titles are on the About page.