British standard system is the system which is based on ISO systems. In this standard system, there is a total of twenty grades of tolerance. The eight grades cover sizes up to 550 mm and the twelve cover set grades up to 3120 mm. The tolerances in these systems for which each grade varies over the range of sizes which have to be produced as soon as possible and the same standard for relative accuracy in each size.
Take an example when the tolerance in grades which ranges from 0.014 mm of sizes below 5 mm to 0.350 mm for sizes between 2600 mm and 3200 mm are exact parity for each size which may be impossible for it to achieve under all these conditions and all the tolerances in each grade will represent precisely the same degree of manufacturing and accuracy which is functional.
The methods of production will vary but the production for a given grade will be unlikely to be the same throughout the ranges.
British standard system will employ tolerance grades which will vary from IT 6 to IT 17 for some specified standard limits and fits. These will depend on the combination of different holes which are of different sizes and shafts and their tolerance grades, which is around 4000 different fits combinations are formed.
This theory is based on theoretical concepts. But in some practical cases, it is difficult to combine every hole with every shaft. For these systems, the deviation which is supposed to be fundamental is the difference between the nominal sizes and the nearest limit for all the tolerances available in the system.
Holes in this system are represented by capital letters and the shafts are represented by small letters. Holes symbolically are represented by 'H' and shafts are represented by 'S'. In this system when these holes are positive and shafts are negative the deviations take place for 'A' and 'a' which is maximum. The holes present in the system from 'A' to 'H' will be directly matched with the shafts from 'a' to 'h' but the remaining groups of holes will not very much.
International Standard System is a system of fits and limits which is derived from the international standards organization which is located in Switzerland and Geneva. This system of limits will help us to understand the limits and fits which have 25 grades of tolerances.
These tolerance grades will be determined and analyzed by analysts all around the world. The tolerance factors present in the system will also be found out and determined. The tolerance grades are supposed to be the product of fundamental tolerance factors.
In the Indian Standard Organisation systems which specifies 28 types of holes and shafts respectively, has represented a wide range of sizes. This system will recommend the use of unilateral holes for the possible suitable combinations for fits to an adequate range of shafts. This system will indicate capital letters for holes and small letters for shafts.