XS: is the size to choose for ribbing on size 20 or smaller.

Small: is an all-purpose size capable of filling many fly tying needs.

Brassie: is an ideal choice for wire bodied flies sizes 18-22 and for ribbing on most trout sized flies.

Medium:  is a good choice for ribbing on size 8-12 nymphs.

Large:  is a popular choice for streamers and/or large stonefly nymph patterns.