Many fly patterns require hair that is perfectly even at the tips. Most hair grows on animals in random lengths.
A stacker enables you to bring all of the hair tips to the same plane quickly. Just insert the hair bundle tip-first into the barrel of the stacker and bang the base of the stacker on the table. The hair fibers will slide until they stop at the bottom of the stacker. Turn the stacker horizontal and remove the base. The exposed hair tips will be even.
Remove the evened hair bundle from the stacker and tie it on the fly. To make this process easier; comb or stroke out, all short hair and under-fur from the hair bundle before it is inserted into the stacker. Straight hair stacks easier than kinky hair. If the hair is greasy or full of static cling, saturate the hair-piece with talcum powder and fluff out.