Definition
A side stack is the formation in which four offensive players hold a vertical line tight to one sideline, opening up the central and far-side space for an isolated cutter (the Iso) and the handler set behind the disc.
In Context
The side stack is the structural piece of our default Spread Stack system. Pinning four players to the sideline does two things: it removes them from the central working space, and it forces their defenders into a choice — stay on the stack and let the iso work alone, or leave the stack and gift a free reset.
The discipline of the side stack is in the holding. A player drifting infield from the stack collapses the central space they were creating. A player who clears properly back into the stack rather than across the field keeps the structure intact for the next cut.
See The Spread Stack - Core for the full system and Vertical Stack for the alternative central-stack shape.