FOCUSED:
A focused grid is one in which the lead strips are tilted progressively as they move away from center.  Lines through each of these strips would converge at a point known as the grid focus. 

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CRISS-CROSS GRID:
A criss-cross grid has two grids sandwiched together with the result that the strips are usually both parallel to and at right angles to the long dimension of the grid.

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PARALLEL:
In the use of a parallel grid, no matter where the tube us located, there is never a condition under which all of the strips will be aligned with the tube, since they are all vertical.  Only the strips directly under the x-ray tube can be in true alignment with the primary rays. It is not recommended to use a parallel grid at any distance less than 56".

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GRID RATIO:
The ratio of a grid is defined as the relation of the height of the lead strips to the distance between them.  The higher the ratio of a grid the more scattered radiation is absorbed.  As grid ratio increases, the necessity of having the focused grid exactly centered and perfectly level under the x-ray tube becomes more and more important.  Also becomes more necessary to use the grid at its focal distance from the tube, instead of being able to use it through a range of distances.  
RATIOS AVAILABLE: 5:1, 6:1, 8:1, 10:1 & 12:1

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FOCAL RANGE:
MDM x-ray grids are available in a variable range of focal distance. With the lower ratio grids the focal range is considerable wider, with the higher ratio grid it is considerably narrower.
Focal distance available:
Small: 26-32"
Medium: 36-40" and 34-44"
Long: 40-72", 48-72" and 60-72"