There are quite a few grids available and I do not claim to be an expert. This is a beginners guide to grids, feel free to comment or correct me if I have got any information wrong.
There are three main manufacturers of grids : PCA, Pergamano and Siesta.
Traditional grids are mesh in a plastic frame, they come in regular and fine grade with a special perforating mat which fits in the frame. The grid enables you to produce accurate perforations. These are made by Pergamano and Siesta.
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PCA
Steel flexi-duo grids seem to have advantages over the older mesh grids. You can emboss dots for example and the the holes are easy to see and line up. I prefer the A4 size and I love the way the perforations fix the parchment in place whilst you are working.
Pergamano
P
ergamano also make steel grids, straight and diagonal, however they are not interchangeable with PCA grids. For example if a design specifies using a Pergamano diagonal grid a PCA diagonal grid will not work in the same way.
Siesta have a wide range of grids for specific borders and patterns and motifs.