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ISO 4378-4:2009 defines basic symbols for use in the field of plain bearings. Additional signs are also defined for use as superscripts and subscripts.

The characters employed are drawn from the Latin and Greek alphabets, Arabic numerals and other signs, for example points, commas, horizontal lines or asterisks. In the simplest case, an application symbol consists of the basic character alone; in the most complex, of the basic character with subscripts and superscripts (additional signs). For the purposes of international applicability, all basic symbols and additional signs have been derived from English words, and designations used in technical literature up to now have been adopted as far as possible. Wide conformity of the symbols for all types of plain bearings has been attempted.

This classification is established for use in calculations and technological and geometrical determinations, as well as in the quality assurance of plain bearings.