hey, i've been reading a few threads on here and wats this backbone thing on blanks. Is it a pre defined line on the rod or is it something you mark out to guide you in placing your guides? Cheers guys

It's how much final blank strength that is found in the butt section.

I'm guessing you're referring to the spine (or spline). If so - it's a result of the manufacturing process that produces a line along the rod that is slightly less flexible. Opinions range on what to do with it compared to guide placement, but I subscribe to the "ignore it" theory, and instead build along the straightest axis.

Has nothing to do with the spine of the rod, it's simply the power the blank exhibits when bent under a load. If two medium power blanks are compared and one has more power than another under load it has more backbone.