What is the 2-item (pair) variant of PAO, and when is it used?

Use only two slots — typically Person of item 1 + Action of item 2 (dropping the object) — packing two items per image instead of three.

* Pairs give 26 images per deck; PAO triples give ~18 — fewer images, but denser scenes. *

Not everyone runs full triples. A pair scheme fuses two items: Person from the first, Action from the second. It's simpler to decode and is the basis of common card methods (e.g. cards handled two at a time). Trade-off: pairs give 2 items per image (26 images for a deck) versus triples' 3 per image (~18) — less compression, but a smaller mental juggling act. Choose by how much complexity you want to manage.

Go deeper:

• Dominic system (Wikipedia) — the canonical Person+Action two-slot scheme.
• Peg systems compared (Wikipedia) — situates pair vs full PAO among peg systems.