It's called statute. State law:
65/3. Requisites for transfer.
(a) Except as provided in Section 3a, no person
may knowingly transfer, or cause to be
transferred, any firearm, firearm ammunition,
stun gun, or taser to any person within this State
unless the transferee with whom he deals
displays a currently valid Firearm Owner's
Identification Card which has previously been
issued in his name by the Department of State
Police under the provisions of this Act. In
addition, all firearm, stun gun, and taser
transfers by federally licensed firearm dealers
are subject to Section 3.1.
http://www.ilga.gov/legislation/ilc...tName=Firearm+Owners+Identification+Card+Act.