 Welcome to this module on datum precedence and constraining degrees of freedom. Datum precedence and constraining degrees of freedom are important concepts because they help control the function and relationship of manufactured parts. Datum precedence is a fundamental principle in the design and manufacture of mechanical parts and assemblies. It refers to the order of importance assigned to different datums, which are specific points, lines, or planes on a part, which are used as references for measurement and geometric tolerances. When designing a part, engineers must determine which datums are most critical to its function and then use them as the primary references for dimensioning and tolerancing. This establishes a hierarchy of datums called the datum reference frame, which is used for the manufacture and inspection of the part. Degrees of freedom refer to the possible movements that a part can have in three-dimensional space. There are six degrees of freedom, three translational which govern movement in a straight line along the x, y, and z axes, and three rotational degrees of freedom, which control the rotation about these three axes, referred to as u, v, and w. Let's look at each of these three degrees of freedom. Translation along the x-axis. The ability of an object to move back and forth in a straight line along the x-axis. Translation along the y-axis. The ability of an object to move forward and backward in a straight line along the y-axis. Translation along the z-axis. The ability of an object to move up and down in a straight line along the z-axis. Rotation about the x-axis. The ability of an object to rotate about the x-axis, referred to as u. Rotation about the y-axis. The ability of an object to rotate about the y-axis, referred to as v. rotation about the z-axis. The ability of an object to rotate about the z-axis, referred to as W. Following datum precedence to establish a datum reference frame, constrains the degrees of freedom of a part, meaning that it limits the ways in which the part can move or be positioned relative to its intended function. The datums with the highest precedence must be held in a fixed position and orientation first. Datums with lower precedence constrain the remaining degrees of freedom. The degrees of freedom able to be constrained by any datum depends on the type of datum. We'll look at this in a moment. It's important to follow the datum precedence when inspecting a part feature or characteristic to ensure the part will function correctly and meet its intended design requirements. To understand how datum features constrain the degrees of freedom, we will start with a planar feature type. With this part, a plane locks in the bottom surface of a part, which prevents its translation in the z-direction. However, the part can still translate in the x and y-directions. The part can also rotate about the z-axis, but not about the x or y-axes. A plane therefore constrains three degrees of freedom, one translational and two rotational. Next, let's look at a cylinder. This prevents translation in the x and y-directions, but not the z. It also prevents rotation about both the x and y-axes. However, the part can still rotate about the z-axis. A cylinder therefore constrains four degrees of freedom, two translational and two rotational. Lastly, let's consider a sphere. A sphere will lock down motion in the x, y, and z-directions, which constrains three degrees of translational motion. However, a sphere does not prevent rotation. Finally, let's look at the datum precedents for the positional tolerance shown here. B, a planar surface, is listed first, so it is the primary datum. It constrains two rotational degrees of freedom, u and v, which are rotation about the x and y-axes, and one translational degree of freedom in the z-direction. The secondary datum, plane A, constrains an additional degree of rotational freedom, which is rotation about the z-axes, or w. It constrains translational freedom in the x-direction. Our third, or tertiary, datum only needs to constrain the sixth degree of freedom, which prevents translation in the y-direction. This completes the module on datum precedents and constraining degrees of freedom.