sharp and smooth usage

Hello I’m trying to figure out how to smooth the diagonal line but not the parallels verticals.
I used “ctrl + E” mark sharp and them smoothed all faces, and it did not leave the marked sharp sharp!
any suggestion welcome,

Mark Sharp is used in combination with the Edge-Split modifier

or you could use flat shading, and localised smooth shading by selecting the face(s) with the diagonal line and W / Shade Smooth

Thanks Richard! I did not yet use much this modifier, I’ll try it more. The other method might be more indicated in this case.

I found how to use split edge! thanks to you Richard!