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International standard ISO 2848 (Building construction – Modular coordination – Principles and rules, International Organization for Standardization, 1984) is an ISO standard used by the construction industry. It is based on multiples of 300 mm and 600 mm
While those dimensions equate to 30 cm and 60 cm respectively, it is the use of millimetres which is significant.
The numbers 300 and 600 were chosen because they are preferred numbers due to their large number of divisors – any multiple can be evenly divided into 2, 3, 4, 5, 6, 10, 12, 15, 20, 25, 30, etc., making them easy to use in mental arithmetic. This system is known as "modular coordination". A related standard is British Standard 6750.
The standard unit of ISO 2848 is a basic module, a length of 100 millimetres (3.937 in) which is represented in the standards by the letter M. Adherence to the standard means that major dimensions such as grid lines on drawings, distances between wall centres or surfaces, widths of shelves and kitchen components are multiples of the basic module. As dimensions increase, preference is given to lengths which are multiples of 3 (see metric foot), 6, 12, 15, 30 and 60 basic modules. For smaller dimensions, the submodular increments 1⁄4 M (see metric inch) and 1⁄2 M are preferred.
A metric foot is only a nickname for a preferred number length of 3 basic modules (3 M), or 300 millimetres (11.811 in). The 300 mm (30 cm) metric rule is of a similar length to the traditional imperial one-foot rule. A metric foot is 4.8 millimetres (0.189 in) shorter than an imperial foot.
A metric inch is a nickname for a preferred 1⁄4 subdivision of an ISO 2848 basic module, or 1⁄12 of a metric foot measuring 25 millimetres (0.984 in). A metric inch is 0.4 millimetres (0.016 in) shorter than an inch, since the inch is defined as 25.4 millimetres.
The term was similarly used to refer to the historical Soviet Bloc practice of spacing integrated circuit pins at 1⁄10 of a 25 mm "metric inch" length, instead of the western practice of 1⁄10 of an imperial inch.