I couldn't hope to be as good a woodworker as Norm, but the lines he calls grain I call the pattern of growth rings in the end grain. I was always under the impression that the grain direction is synonymous with the direction of the longitudinal fibers that run the length of the board. My point is, if you cut your splines with the fibers running the length of the spline, I believe it will be weaker than if you have the fibers perpendicular to the length of the spline, whether the you cut them plain sawn or quarter sawn. Norm's saying plain sawn won't split, whereas quarter sawn will. An old book I have on boat building shows splines cut with the fibers perpendicular to the length of the spline. Wood splits by forcing the fibers apart the way you tear a corn husk lengthwise. Try to tear a corn husk crosswise - you can't do it.
I couldn't hope to be as good a woodworker as Norm, but the lines he calls grain I call the pattern of growth rings in the end grain. I was always under the impression that the grain direction is synonymous with the direction of the longitudinal fibers that run the length of the board. My point is, if you cut your splines with the fibers running the length of the spline, I believe it will be weaker than if you have the fibers perpendicular to the length of the spline, whether the you cut them plain sawn or quarter sawn. Norm's saying plain sawn won't split, whereas quarter sawn will. An old book I have on boat building shows splines cut with the fibers perpendicular to the length of the spline. Wood splits by forcing the fibers apart the way you tear a corn husk lengthwise. Try to tear a corn husk crosswise - you can't do it.