National Instruments Xmath Interactive Control Design Module ICDM - Manual (Page 31)

Chapter 2 Introduction to SISO Design
Xmath Interactive Control Design Module 2-16 ni.com
To add a pole-zero pair, click the Add Pair button, select the Add Pair
entry on the Edit menu, or press <Ctrl-P> in the window. As with poles
and zeros, the pole-zero pair you create will be either real or a complex
conjugate pair, depending on how close the cursor is to the real axis
when you click the left mouse button.
After the pair is created, you can drag the pole and zero away from
each other, which results in a smooth change to the transfer function.
By convention, the cursor first grabs the zero in a pole-zero pair.
To delete a pole and zero that are very near each other, click the Delete
button, position the cursor near the pole and zero, and click the left mouse
button. This will remove the pole and zero but have little effect on the
transfer function.
If you want to delete a pole or zero that is very near a zero or pole,
respectively, then you may have to first separate them a little bit. Otherwise,
the delete command may be interpreted as a delete pair command.