The polyester and adhesive content in hot laminating films is defined as a ratio, such as 1/4 or 2/3. The numbers are usually in mils, so these ratios would be for 5 mil film. The first number represents the thickness of the film layer (usually polyester), while the second represents the thickness of the adhesive layer. Films with a higher polyester number are stiffer and provide greater protection and durability. Films with a higher adhesive number will have a stronger bond, which is good for laminating thick or difficult materials.