Ill. Admin. Code tit. 86 § 130.725
The taxpayer must conspicuously display his certificate or sub-certificate of registration in the place of business to which it applies. Wherever possible, the taxpayer must display his certificate or sub-certificate of registration in the place of business to which it applies by affixing such certificate or sub-certificate to the glass part of a window or door which faces or opens upon a public street or highway, or in some similar position. If no window or door faces or opens upon a public street or highway, the certificate or sub-certificate must be posted in some conspicuous place which is as near as is practicable to the entrance of the taxpayer's establishment, and such certificate or sub-certificate must be exposed to public view.
Ill. Admin. Code tit. 86, § 130.725
Amended and effective September 9, 1969