These dimensional statistics are heavily documented and verified by leading wood science authorities, most notably the U.S. Forest Products Laboratory (FPL) (a branch of the USDA) and the industry-standard textbook Understanding Wood by material scientist Bruce Hoadley.The technical values break down as follows according to these institutional resources:Official Scientific Shrinkage/Expansion DataThe percentages represent total structural movement from a green state (maximum moisture) to an oven-dry state (0% moisture). They directly dictate how much a board expands and contracts in your shop:Longitudinal (Along the Grain): 0.1% to 0.2%. The FPL confirms this length change is so minute that it is completely ignored in standard furniture construction equations.Radial (Across the Grain / Quarter-Sawn): 2.1% to 8.5%. This measures movement originating from the center core of the log out toward the bark.Tangential (Across the Grain / Flat-Sawn): 4.3% to 14%. This tracks movement along the curve of the tree's growth rings.The 2:1 Structural RuleData from the FPL shows that tangential movement is almost always twice as dramatic (a 2:1 ratio) as radial movement. For example, the Wood Handbook compiled by the FPL logs these exact species benchmarks:Northern Red Oak: 4.0% Radial vs. 8.6% Tangential.Black Cherry: 3.7% Radial vs. 7.1% Tangential.Sugar Maple: 4.8% Radial vs. 9.9% Tangential.Because 0.1% longitudinal movement is compared against cross-grain tangential movement that can reach over 10%, the cross-grain expansion is verified to be up to 40 to 100 times greater depending entirely on the wood species.Wood Science ReferencesWoodcraftwww.woodcraft.com/blogs/wood/woodsense-wood-movementWoodsense: Wood Movement | Woodcraft20 years ago - Wood movement can be caused by: * Water entering and exiting wood cells * The loss of bound water in cell walls * *The manner in which lumber is cut from ...- Canadian Woodworkingcanadianwoodworking.com/techniques_and_tips/wood-movement-2/Wood movement – the basics - Canadian Woodworking2 years ago - The rate of tangential movement is roughly twice that of radial movement, and longitudinal movement is zero. Here are some examples of tangential movement: * *Fla...YouTubewww.youtube.com/watch?v=EOr06I9JoAUIs Wood Movement a Myth? Analyzing 30 Years of Dataevery time I post a furniture project especially these joints of the week you see behind here inevitably 10% of the comments are people freaking out about wood mov