Datum Reference in Drawings

http://www.qmsi.ca/datumdevelopment.html

Primary datum controls 3 degrees of freedom; the translation along the Z axis, rotation about the x axis, and rotation about the y axis. Secondary datum controls 2 degrees of freedom; translation along the y axis and rotation about the z axis. The tertiary datum controls 1 degree of freedom, translation along the x axis.

Learn here.

Primary = 3 points of reference or (Level)

Secondary = 2 points of reference or (Align)

Tertiary = 1 point of reference or (Origin)

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A "3,2,1" alignment achieves Level, Align, Origin (which establishes X0,Y0,Z0)

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Think of a 3,2,1 alignment as establishing home plate location at the baseball field. Your going to report/design the location of the other bases, dugouts etc.... from that primary location. If I need to know where first base is from home plate (X0,Y0,Z0) then you would give me coordinates from (relative to) home plate.

The objective is to lock - or - establish X zero, Y zero, Z zero.

** Usually but not always defined as:

X = side to side movement

Y = front to back movement

Z = up and down movement

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Example: Lay a framing square on a work bench and place a block of wood or metal in the corner. You have just created a "nest" that with a clamp isolates 6 degrees of freedom. x,y,z, - pitch, yaw, roll about these 3 axis.