Hi Dimitra! This is a tricky part of the definition. The whole reason we're doing this is to sort our edges according to the intersection lines/Breps. Ok, so remember we divided the individual edges into 4 points to check for inclusion into the Breps. This will result in 16 numbers and we partition them back to having 4 sets of numbers per edge. So far so good. Now in this next part, I just want to check 2 things: 1) if a point lies in the Brep, and if so 2) what index that Brep is. I need that info to sort my intersection lines accordingly which occur later on in the lesson. Now for the first part, if one of those 4 points lies within a Brep it will result in a value above 0, else it will return -1. So when I have -1, -1, -1 -1; this means none of the points lie within a Brep, therefore there is no intersection line. If for instance we have -1, 0,0, -1. This means the outer two points lie outside the Brep, but the inner points lie within a Brep. And the index of that Brep/intersection line is 0. We only need one value per edge, not 4. By sorting the 4 values and getting the highest value, we make sure that we get the index of the Brep/intersection line. In our example: -1, -1, -1, -1 ---> results in -1, meaning no intersection line for this edge. -1, 0 ,0, -1 ----> results in 0. Meaning yeeeey we have an intersection line/Brep with index 0. I hope this makes it a little bit more clear Good luck! Arie