... to extend the length of them to see where this is happening. This also means that if they are both curving in the same direction they cannot both be the same radius. It is mathematically impossible to create a transition curve if the centre-lines intersect. You need to adjust the radius of one or other of them until there is always a gap between them, i.e. they nowhere touch or cross over. The gap can be very small but it must exist. Here is what I mean by a gap: 2_231823_100000000.png The terminology for transition curves is explained on this page: http://templot.com/martweb/gs_geometry.htm#transition I'm sorry you are having problems with this, but it is all explained in the program if you click the Help buttons, and also on this page of the web site (sorry it is so old): http://templot.com/martweb/info_files/make_trans.htm Here for example are the Help notes copied and pasted direct from Templot: Make Transition Curve from Control Template to Background Template For diagrams ...