Builders disagree and both camps publish arguments: find the spine and align guides with it for power, or ignore it because modern blanks vary little. No test settles it, so decide deliberately rather than by accident.
Roll a blank tip-heavy on a hard surface and it settles with the heaviest sector down — the spine (or spline) is that seam, an artefact of how the carbon cloth wraps during manufacturing. RodMaker Magazine's reprint and the retailer guides describe the same roll-and-find procedure to locate it.
The traditional position: the spine is the stiff axis, so placing guides on or near it puts the rod's strongest plane in line with the load and gives truer casting and fighting behaviour. Builders in this camp locate the spine on every blank and mark it before layout.
The counter-position: spine location wanders even along a single blank, modern mandrel wrapping is consistent enough that the effect is small, and chasing it adds a variable that cannot be controlled precisely anyway. This camp places guides by line path and static test, spine noted but not obeyed.
Even spine-findings agree the effect, where present, matters most on lighter blanks and least on heavy saltwater sticks — which is the opposite of what 'find the strong axis' intuition would suggest, and part of why neither camp has conceded.
Find the spine, mark it, look at where your chosen spacing method puts the guides relative to it, and decide for the rod in your hands. Our Reference gives both positions with their reasoning rather than picking a winner, because the craft has not picked one either.
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