How to display a spatial bar in an image?

Hi All,
I am a newbie in using 3D-slicer. My Q is:
How to display a spatial bar in a image so that I may know the dimensional scale for an image
Thanks a lot,

Hi AJ,

You could display the ruler from the menu available in the 2D slice viewer:


Also available in 3D views (in parallel/orthogonal projection mode)

Thanks a lot, JC. That really works!

Thank you too, Iassoan! I may shoot more questions here since I am a really newbie here and you guys experts here!