scope:

This Standard is complementary to CSA Standard CAN3-B78.11 and covers dimensioning and tolerancing of products depicted on technical drawings.

Some of the technical terms dealing particularly with dimensioning and tolerancing are defined in Clause 3. Abbreviations commonly used on drawings are given in CSA Standards 285, CAN3- 878.1 and CAN3-878.3.

Although separate clauses have been assigned to the general principles of dimensioning and of tolerancing, it should be borne in mind that the two terms are often inseparable, since the value of a dimension as specified on a drawing represents a basic size with a suitable tolerance. Therefore, the term "dimensioning" is frequently used in this Standard to mean the assignment of values for the limits of size or the basic size and tolerance.

Most of the figures in this Standard are intended to illustrate a dimensioning principle, and do not represent completely dimensioned parts. Where numerical values are shown, they are examples only, intended to illustrate the point under consideration, or to clarify the interpretation. Capital letters have been used for those notes which might appear on the drawing, whereas notes in lower case letters are intended only to explain the principle being illustrated. Many illustrations have been reduced for reproduction purposes, and may not meet the size and spacing requirements specified in the text for use on drawings.

For simplicity in showing the interpretation of geometrical tolerances herein, it has been assumed that the feature or features under consideration have no other errors than those with which the interpretation deals. 

In many illustrations of geometrical tolerancing, the size and shape of the tolerance zones are shown somewhat exaggerated. These are for explanatory purposes only, and are not intended to appear on the drawing.

References to gauges in this Standard are intended only to explain the correct interpretation of tolerancing. Such references are not intended to control or limit the design of gauges or restrict the application of alternative methods of ascertaining sizes.

Illustrations in this Standard are in third angle projection, and linear dimensions are in millimetres but the principles involved apply equally to drawings in first angle projection or dimensions in other than SI units.

It is recommended that drawings prepared in accordance with this Standard reflect this in a note, such as "INTERPRET IN ACCORDANCE WITH CAN/CSA B78.2-M91".