How to type an underscore

After at least half an hour and numerous restarts trying to walk someone through how to install Skype over the phone, we begin the arduous process of adding contacts:

Me: Do you know what an underscore is?
Nube: No.
Me: Do you see that button with two lines on it.
Nube: No, where?
Me: It's next to the + sign
Nube: It has two lines underneath the + sign
Me: No, that's the equals sign. The one you're looking for is next to the + sign.
Nube: To the right or left?
Me: To the left
Nube: Okay, that button has a big line and a little line
Me: Press the shift button at the same time as that button with the two lines on it
Nube: Oh.