@dave r said:
Unfortunately your screen shot really doesn't show us what you are wanting to do. Have you tried turning on Hidden Geometry so you can see those edges? What is you want to do after selecting these edges?
By the way, are you still using SketchUp 2015 Make as your profile indicates?
I apologize for the misunderstanding. here is another screenshot showing the hidden horizontal lines that I want to select without the diagonal lines in order to add horizontal mullions. Thanks