Both ruler and protractor tools share the same key -
7. Press it one more time to switch between the two.
Ruler is used to measure distances between arbitrary points. There can be many rulers present in the scene simultaneously.
To place a ruler click
LMB on both ends of a segment you want to measure. The length of the last placed ruler becomes a reference length. The new rulers will snap to this reference length.
Draw tool can snap to rulers the same way it snaps to object edges and other lines.
If you click on an existing ruler with
LMB, its length will be set as the reference. To delete an existing ruler click it using
RMB elsewhere resets the reference length.
Protractor is used to measure angles between lines and planes in any combination.
To measure an angle click
LMB on any line or plane, then hover the cursor over any other line or plane to see the angle, or click
LMB to set this angle as a reference one. Click
LMB one more time to measure another angle.
If the reference angle is set, the draw tool will be able to snap to its value using angle snapping mode.