A Stack block is a rectangular block that is shaped to fit above and below other blocks. Stack blocks make up the majority of the blocks available.
Stack blocks are fitted with a puzzle-piece like shape; the top has a notch and the bottom has a bump. Because of this shape, scripts can stretch on and on - the block tessellates.
Their shape allows them only to be placed in the following areas:
As Stack blocks are shaped to have blocks above and below them, they are used almost everywhere in a script; scripts must always have a Stack block in them (unless you put a Cap block directly below a Hat block). An example script:
Note how the Stack blocks are used in the script - they make up the major commands. (stop script included.)