Ill. Admin. Code tit. 14 § 470.210
It is an unfair or deceptive act for a seller to make a price comparison or claim a savings as to any product it offers (for example: "$29.99 - Save $10.00 " - or - "20% Off all men's shirts") unless the seller clearly and conspicuously describes the basis for the price comparison or the savings claimed; provided, however, a seller may compare a higher and a lower price without describing the basis for the price comparison or the savings claimed if the higher price is the seller's own former (regular) price as determined in accordance with Section 470.220 of this Part. Terms such as "formerly," "regularly," "originally," or words of similar meaning may be used by the seller to identify the higher price as its own former (regular) price.
Ill. Admin. Code tit. 14, § 470.210