Earlywood and Latewood

Every annual growth ring is actually two distinct bands of wood: earlywood (also springwood), grown quickly early in the growing season, larger-celled and less dense; and latewood (summerwood), grown more slowly later in the season, smaller-celled and considerably denser. The visible contrast between the two -- pale, soft earlywood next to darker, harder latewood -- is what makes individual growth rings visible at all, and why ring-porous species like oak show such a strong, almost striped grain pattern compared to a diffuse-porous species like maple, where the earlywood/latewood contrast is much subtler.

The density difference matters practically, too: latewood is harder and more resistant to hand-tool cutting than the earlywood right next to it, which is part of why species with a strong earlywood/latewood contrast can feel like they're fighting a hand plane inconsistently within a single stroke.