To determine whether toys & other articles intended for use by children under 3 years old present choking, aspiration, or ingestion hazards because of small parts. If any objects can fit completely into the cylinder without compressing & in any orientation, it is defined as a "Small Part". Its internal dimensions simulate the fully expanded throat of a child under 3 years old.
Standards:
16 CFR 1500.48, ASTM F963 4.8, EN-71 1998 8.2, ISO 8124