Stack Block


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.

Execution Process

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:

Controlling the LED brightness according to the ultrasonic value.


The entire execution process for this stack of stack blocks will take approximately 0 seconds. Opposed to visualizing the sprite move 5 steps, then 4, etc. the entire motion will be seen in one unified step; the end user will witness what appears to be the sprite move 15 steps, even though it has 5 separate blocks making up the motion. The elapsed time variable will be 0 when the script finishes executing.


Extremely long and intensive scripts could take a long time to process and may actually have a notable enough elapsed time.

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 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.