16 C.F.R. § 300.26
The fiber content of pile fabrics or products made thereof may be stated in the label or mark of identification in such segregated form as will show the fiber content of the face or pile and of the back or base, with the percentages of the respective fibers as they exist in the face or pile and in the back or base: Provided, That in such disclosure the respective percentages of the face and the back be given in such manner as will show the ratio between the face and the back. Examples of the form of marking pile fabrics as to fiber content provided for in this section are as follows:
100% Wool Pile
100% Cotton Back
(Back constitutes 60% of fabric and pile 40%)
Pile-60% Recycled Wool, 40% Wool
Back-70% Cotton, 30% Rayon
(Pile constitutes 60% of fabric and back 40%).
16 C.F.R. §300.26