Heading 6501 covers felt forms that are neither blocked to shape nor fitted with a made brim. Once the blank has been blocked into a definitive hat shape or a brim has been added, it advances to the appropriate finished headgear heading (such as 6504 or 6506), even if it still lacks lining or trim.

Felt for this heading is a non-woven textile made by matting, condensing, and pressing fibers — typically wool, fur, or a blend. The felt must be of the hat-body type; woven or knitted fabric formed into a hood-like shape does not qualify and would classify under a different headgear heading.