One thing I have noticed is that you can change the size of the ruler by dragging the handle at the bottom right of the ruler. This is something I don’t want the students to do when they are measuring several lines because it changes the value of the units. Therefore, I always add the ruler to the page and immediately right click or click on the drop down menu and choose Locking. From the locking menu, I click on Allow move and rotate. Then the students can still use the ruler but they will not alter the unit value.