Thank you very much michael, I have been practicing and I realized that you have to plan the creation of the object so that when applying chamfer the lines do not overlap, although in polygonal modeling you have to plan the direction of the polygons to have a correct flow to When applying the subdivision, in NURBS I have realized that it is not modeling for modeling either. Now I had this problem and I don't know how to solve it, I have this line and those two vertices are separated, logically when I extrude it doesn't create a volume since the shape is open, how can I weld those two vertices so that they collapse in their center and solder?, Thank you and sorry for bothering you so much.