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 in Scratch, being available in every category except Operators.
When 2 stack blocks are connected to form a script their commands will execute in the order from top to bottom. The entire collective stack executes in a single frame. For example, take the following script:
Stack Blocks With Delay
Some stack blocks do execute with a delay, meaning there may be a pause between it and the next block executing. The following blocks have this behavior:
Stack blocks are fitted with a puzzle-piece like a 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.
As Stack blocks are shaped to allow blocks above and below them, they are used almost everywhere in a script; scripts must always have a Stack block in them to be functional.