Lower second premolars – #20 and #29
The second premolars on each side of the lower jaw, just in front of the first molars.
Roots
Usually one. Two roots are uncommon (about 4% in pooled Saudi studies).
Canals
Usually one (92% in pooled Saudi studies). A second canal turns up in about 5% worldwide.
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 11–12 years old, according to the ADA's eruption chart.
They replace the lower baby second molars, which fall out at 10–12 years.
They bite against the upper second premolars.
Sources
Mashyakhy et al. 2022; Martins et al. 2021; American Dental Association 2012; Harris 2005. Full titles are on the About page.