# Datum Principles

## Datum Elements and Datum Systems

Datum elements and datum systems are only specified in case of geometrical tolerancing (not on dimensional tolerancing and except on form tolerancing). You can specify simple datum elements, common datum elements, datum targets or specified datum systems.

The toleranced feature is positioned relatively with the tolerance zone, and this set is positioned relatively with the datum or the datum system. These related positions will be specified using basic dimensions and they are displayed or not.

For more information about datum constitution and specification, see ISO 5459, ASME Y15.5M and ISO 1101.

## Datum System Composition

When the identifiers are specified separately in each frame of the tolerance frame, the datum elements represent a datum system. A hierarchy is established between the datum elements. Reference A is the primary datum and reference B is the secondary datum. The datum system fitting would be performed first on datum A, then on datum B, with respect to A.  By the way, a tertiary datum can also be specified.
The datum elements have to be fitted successively (following their specification order) with respect to the hierarchy for the measurement computations.
When only one identifier is specified in the tolerance frame, the datum is a single datum.
When two identifiers separated by a dash are specified in the tolerance frame, the datum is a common datum. The two datum elements are to be considered simultaneously. The datum system fitting would be performed in the meantime on datum A and on datum B (both datum elements have to be fitted simultaneously for measurement computations).
These symbols represent two specifications: A | B and B | A.

## Datum writing rules

A capital letter is used to identify the datum element in the tolerance frame. The datum triangle may be filled or non-filled.

When the datum triangle is placed on the outline of the element or on its extension line, the datum element represents the surface itself or the 2D representation of the surface, which is a line.

When the datum triangle is placed in the alignment of the dimension line, the datum element represents the median element (usually an axis or a median plane).

When the datum triangle points directly on a median element, the datum element represents either the median element itself (usually an axis or a median plane) or the resulting median element of the collection of the considered elements.

The tolerance frame may also be related to the datum element using a leader line.

When the tolerance frame is related to one datum element through a leader line, the datum identifier may be omitted in the tolerance frame.