It's a #10 is one of the best-selling essential brushes. 

Benefits:

• It has a firm, flat synthetic bristles that make it ideal to line the eye with exacting precision. 
• It can be used with powder, liquid, or cream products. 
• It’s specifically designed to applying the product in an exact, fine line to the base of the lower or upper lashes, but it’s supple enough to blend shadows into the lash base.
• It can be used dry to apply shadows to the lash base or can be used wet with Transforming Gel and a cake eyeliner, eye shadow, or shimmer to create a waterproof, custom liner.