Lower first premolars – #21 and #28
The first of the two premolars on each side of the lower jaw, just behind the canines.
Roots
Usually one (87% in pooled Saudi studies); about 13% have two.
Canals
Usually one (75% in pooled Saudi studies). About 1 in 4 has a second canal, in both the Saudi and the worldwide data.
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 10–12 years old, according to the ADA's eruption chart.
They replace the lower baby first molars, which fall out at 9–11 years.
They bite against the upper first premolars.
Sources
Mashyakhy et al. 2022; Martins et al. 2021; American Dental Association 2012; Harris 2005. Full titles are on the About page.