Double rows of vertices?

[JimB_again]

4) Related to the above, I've tried to cut segments off a model to in order to keep nothing but that which will show. I'm unable to use the "knife" though. I click "knife", "slice surfaces" and then I need to "set" knife. "Line" comes up and then "knife: knife cannot slice itself -- please deselect the knife". All right, I'm clueless; how do I perform the simple task of slicing an object in two and discard the part that I don't want to keep?

I have just purchased the 6.5 upgrade and I cant get the "Knife" tool to work either,please can someone shed some light on this problem?