Heartwood
Also called: duramen
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Heartwood is the older inner wood of a tree, usually distinct in colour from the younger sapwood around it, and it often matters when decay resistance is important.
Heartwood and sapwood
Sapwood is the younger outer wood, while heartwood lies farther towards the centre. The boundary and colour vary between species. In white oak, freshly cut heartwood can be russet, orange-brown or reddish brown, sometimes with darker stripes. Colour alone is not a structural rating.
Strength and durability are separate
Rivington (1875) regarded heartwood in most trees as stronger and more durable than sapwood. Hoadley (1980) separated the two properties, reporting that basic strength does not change when sapwood becomes heartwood. Use the distinction carefully: heartwood can be preferred for decay resistance in a particular species without every heartwood board being stronger for every load.
A specified use
White-oak heartwood is often chosen for ship planking and bent boat parts because of its decay resistance. That is a species-specific use, not a rule that all heartwood is decay-proof.
Movement during drying
A board containing inner and outer wood can finish unevenly if those zones contract by different amounts as they dry. Allow for that movement when choosing stock and laying out a joint, especially where a flat or accurately fitted surface matters.