The "X's" on a cowboy hat refer to the percentage of beaver fur used in felt cowboy hats. A 4X hat contains 4% beaver fur, and hats with more "X's" contain higher percentages of beaver and cost more. Pure beaver hats contain 100% beaver fur and are the most expensive. The quality of the beaver fur used also impacts the price, with brands like Resistol and Stetson using higher quality parts. Wool cowboy hats do not contain beaver and are cheaper and more durable than felt hats. The tightness of weaving also affects the price of straw cowboy hats, with higher "X" straw hats being more tightly woven.