I guess I just don' understand the math. Where is the radius of a curve measured? The inside edge of the curve? The center?

The pieces I'm looking at now are banked turns and as I mentioned, I see a 9", 12" and an 18" available. I'm wondering if this means for a 4 lane track I can do either a 9 inside of a 12 or a 12 inside of an 18. Am I on the right "track" here?

Thanks for the help

Brian A