Lower first molars – #19 and #30
The first big back teeth on each side of the lower jaw.
Roots
Two roots, one toward the front and one toward the back. An extra third root turns up in about 6% of these teeth worldwide, and in up to 22% in China.
Canals
Three or four. In pooled Saudi studies, 59% had three canals and 41% had four.
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 6–7 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 first molars.
Sources
Mashyakhy et al. 2022; Martins et al. 2022; Hatipoglu et al. 2023; American Dental Association 2012; Harris 2005. Full titles are on the About page.