What are paint brush bristles made of ?

At the moment, paint brush bristles have two kinds, one is bristle, another one is pure synthetic.

If it is bristle, it would be pure bristle or bristle mixture synthetic, natural bristle is more durable and good painting holding and release, Natural Bristle is more suitable for oil-based painting. Bristle mixture synthetic is also better in oil-based painting. Natural bristle is more suitable for outdoor painting.