But see, they are still measuring ' flex.' Aluminum arrows don' t really have much of a ' spine' because they flex pretty much the same no matter where you rotate the shaft to check the flex. On woods and carbons, there are pretty hefty differences from one point around the shaft to the next. Where the stiffest line running lengthwise along the shaft is, that' s the actual spine of the shaft.

Our use of the term ' spine' to describe ' flex' is a holdover from wood arrow days. The spine of a wood arrow runs with the grain and that is where you test the flex. We archers just came to call it spine. Incorrectly, I might add.