What bookdealers really mean. A dictionary by Tom Congalton and Dan Gregory (Between the Covers), with additions from other sources
Although this is a specific sort of plastic film, in the book trade it is used as a generic term to indicate one of several types of plastic used to protect books and dustjackets. These can come in precut sizes for jackets, or in rolls that can be cut and shaped to each individual book. Brodart is one popular manufacturer of these mylar liners, and novices in the book trade sometimes refer to any kind of mylar as Brodart, even if it comes from another company.