3-2-1 Principle of Location used in Jig & Fixtures:
 It is also known as six pin or six point location principle. In this, the three adjacent locating surfaces of the blank (work piece) are resting against 3, 2 and 1 pins respectively, which prevent 9 degrees of freedom.
 The rest three degrees of freedom are arrested by three external forces usually provided directly by clamping.
The 3-2-1 principle states that the six locators are sufficient to restrict the required degree of freedom of any work piece. In this, motion is restricted using clamps and locators. A three pin base can restrict five motions and six pins restrict nine motions
Methodology of 3-2-1 Principle: For this, refer the below figure;
 The work piece is resting on three pins A, B and C which are inserted in the base of the fixed body.
 The work piece cannot rotate about the axes XX and YY and also cannot move downward.
 In this way, the five degrees of freedom 1,2,3,4 and 5 have been arrested.
 Two pins D and E are inserted in the fixed body, in a plane perpendicular to the plane containing pins A, B & C.
 Now the work piece cannot rotate about the Z axis and also it cannot move towards the left.
 Hence the addition of pins D and E restrict three more degrees of freedom, namely 6, 7 and 8.
 Another pin F in the second vertical face of the fixed body, arrests degree of freedom 9.
The above method of locating a work piece in a fixture is called the 3-2-1 Principle.