3-2-1 RULE

The 3-2-1 rule helps us in defining the number of points of a part feature which must come in contact of primary, secondary and tertiary datum planes.

This rule is only applicable on planar datums.

Primary datum feature has mainly 3 points of contact with the datum plane.

Secondary datum feature has at least two points of contact and the tertiary datum feature has at least one point of contact with its datum plane.

The 3-2-1 rule is strictly applicable to planar datum features only.


A datum feature which is inclined other than 90° is known as inclined datum features.

For the parts with datum features (surfaces) at an angle other than 90°, the datum reference frame will contain the planes at the basic angle of the part surface.

Inclined Datum Features
Inclined Datum Features

The above image shows that a surface is 60° from datum feature A; this surface is designated as datum feature C.

The datum reference frame has 3 mutually perpendicular planes.

However, the inclined datum feature would have its datum feature simulator orientated at the basic angles.

In such cases, the angle of the surface must be specified as basic, and the surface is controlled by a geometric tolerance.


At times there is a need for a part to contain more than 1 datum reference frame.

A part may contain many datum reference frames to define its functional relationships.

The datum reference frames can be at 90° or at different angles.

Also, a datum plane can be used in more than 1 datum reference frame.

Datum Reference Frames
Datum Reference Frames


Two or more surfaces that are on the same plane is known as coplanar surfaces.

Two or more datum features which are on the same plane is known as coplanar datum features.

A single datum plane can be formed from multiple surfaces.

The profile control mentioned in the below example limits the flatness and coplanarity of the surface.

Coplanar Datum Features
Coplanar Datum Features

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