FreeCAD: Constraints - Overview

[last updated: 2020-06-22]
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Take what's here with usual disclaimers: it's surely incomplete, may be wrong, but it's my best guess at the moment based on tutorial and forum study and my own experience.
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Constraints: General Explanation:
• FC drawings start as 2d. Entities (lines, circles, arcs, etc.) are added to the drawing without regard to precise location or dimensions or angles.
• As such there's a lot of ambiguity to drawings in their early stages of development. This ambiguity is quantified in the concept of "degrees of freedom" (DOF), meaning, how many ways can the elements in the drawing move. For example, a simple line has 4 degrees of freedom: each end can move freely in both x and y directions. Two random, un-constrained lines in a drawing result in 8 DOF.
• Applying constraints removes the ambiguity. For example, applying a horizontal constraint to the line results in 3 degrees of freedom: each end can move in the x direction, and the entire line can move in the y direction.
  Adding a length constraint results in 2 degrees of freedom, and locating one end of the line to the origin removes the last two DOF's and "fully constrains" the line. It will turn green.

• In general, the process of adding a constraint is:
  Select an element - In the case of the line, you could select either end or the line itself.
        (some constraint types require two elements to be selected)
  Select the constraint icon desired.
  Enter any numeric value required - length, angle, etc.
  When constrained, a constraint icon will be added to the drawing at the element that's been constrained. You can hide these icons (left sidebar under Constraints. click the checkbox on the left of the constraint line)

• Editing & Deleting Constraints:
• On the left sidebar there is a block labeled Constraints. Click the header/title to expand/contract its listings
  Delete a constraint in one of two ways:
  • Click on a constraint listing in the Constraints block, press delete.
  • Click on the far-left checkbox in the constraint listing to "show" the constraint in your drawing.
    click on the constraint icon in your drawing. It will change color. Press delete.
• Edit a constraint: If the constraint has a numeric attached to it (length, angle, radius, etc), double-click on the numeric in your drawing, or double-click on the constraint listing in the left-sidebar, and a dialog box will open where you can change the value.
  Warning: Do Not "re-constrain" the entity (ie. by selecting the entity and then selecting the constraint icon from the menu bar at the top) just so you can change its value. Doing so "over-constrains your model, and you risk problems up to and including crashing.

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