The late wood of all species is denser than that formed early in the season, hence the greater the proportion of late wood the greater the density and strength.
When examined under a microscope the cells of the late wood are seen to be very thick-walled and with very small cavities, while those formed first in the season have thin walls and large cavities.

The strength is in the walls, not the cavities.

In choosing a piece of pine where strength or stiffness is the important consideration, the principal thing to observe is the comparative amounts of early and late wood.

The width of ring, that is, the number per inch, is not nearly so important as the proportion of the late wood in the ring.
It is not only the proportion of late wood, but also its quality, that counts.

In specimens that show a very large proportion of late wood it may be noticeably more porous and weigh considerably less than the late wood in pieces that contain but little.