Lower second molars – #18 and #31
The big back teeth between the first molars and the wisdom teeth in the lower jaw.
Roots
Usually two (97% in pooled Saudi studies). Some have a single horseshoe-shaped (C-shaped) root instead: about 14% worldwide and 44% in China.
Canals
Usually three (87% in pooled Saudi studies).
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 11–13 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 upper second molars.
Sources
Mashyakhy et al. 2022; von Zuben et al. 2017; American Dental Association 2012; Harris 2005. Full titles are on the About page.