Stacker is a skill that can be given to a lemming in Lemmings 2: The Tribes. A Stacker builds a vertical wall consisting of 12 bricks, adding terrain to the level. Once a Stacker runs out of bricks, he will shrug his shoulders and continue walking. You can make him build again by selecting him before he falls off the wall. The lemming will stop stacking if it bumps into a ceiling. The direction in which the lemming faces alternates with each brick placed, meaning the Stacker can be used in conjunction with another skill or with a low ceiling to turn the lemming around.
A Stacker that faces left initially will move one pixel to the right while building. Thus, when multiple Stackers in succession are given to a lemming facing left, the resulting wall is not exactly vertical.