Hi, Dimitra! If you go to https://thinkparametric.com/courses/deconstructing-the-serpentine-pavilion-2016/videos/201 @5:00 Here you can see that the YZ plane is used from the A output. So we have YZ-planes on all the center points of our rectangles. Then we use the "Plane Coordinates" with the input points from our B output. From those points, the only thing we want to know is how far they are above or below the YZ planes. This measure is the Z output from the "Plane coordinates" component. I hope it's a little bit more clear like this. Good luck! Arie