Transparent plates containing uniformly spaced parallel lines in the cross-wise direction. It is possible to determine the number of threads per unit length (cm or inch) in a fabric by selecting an appropriate grating and placing it parallel to a set of threads. The number of lines appearing on the grating indicates the difference between the total number of lines on the grating and the total number of threads in the area covered by the grating. By placing a grating at a small angle to a set of threads, irregularities in their spacing can be detected.
Textile Resource (http://www.textile.org.uk)