Which of the following is NOT a typical layer in tufted floor coverings?

Tufted floor coverings typically consist of a few essential layers that contribute to their structure and functionality. The pile or face yarns form the visible surface of the carpet, providing texture and design. The primary backing serves as the foundational layer that supports the yarns and gives the carpet its shape and stability. A layer of adhesive is also typically used to secure the pile yarns to the primary backing, ensuring durability and maintaining the integrity of the carpet over time.

Secondary tufting, however, is not a standard layer you would encounter in the typical construction of tufted carpets. While tufting is the technique used to insert the pile yarns into the backing, referring to it as "secondary tufting" suggests an additional or separate process that is not commonly identified as a distinct layer in the overall construction of tufted floor coverings. Instead, tufting occurs as part of the primary construction process and does not represent an additional layer in the final product. Thus, this option correctly identifies something that does not fit within the typical layers found in tufted floor coverings.