Specimen support grids are now almost universally 3.05 mm in diameter, except for a few high resolution stages and some very old instruments. They are available in a vast range of materials and designs. One catalogue lists 86 types in a total of 10 materials. The reason for this proliferation is to enable one to control the following:

1. (a) the amount of support the specimen needs (unsupported areas range from 20 pm to 1 mm in extent);

2. (b) the material of the grid, so that it neither interferes with X-ray analysis nor reacts with the specimen;

3. (c) the labeling of specific regions of a specimen (many grids have identification marks for relocation of interesting fields).

The cheapest most widely used supports are copper grids at a spacing of 100 bars in"1. Most grids have a shiny side and a dull side. Opinions differ as to the best side on which to mount the specimen but if a consistent practice is adopted it is always known which way up in the microscope the specimen was mounted.