Grain is the region of where springwood and summerwood resulting from different growth parameters. Basic grain descriptions and types are:
Straight: Tree species growth straight and parallel with the tree's trunk.
Spiral: Tree species grow with the grain at a slight incline, with spiral like circling the trunk.
Interlocked: Occurs when spiral grained trees change directions by spiralling forth then backward, spiralling left then right.
Photo shows the spiral growth changes direction from year to year (growth rings), this is what is called "interlocked grain" that gave images of ribbon stripe (photo below), curly patterns and etc
Typical wood showing very obvious interlocked grain.