mm wrote:

Pressure treated lumber is soaked in the anti-rot chemicals and
expands. When erected, it dries and shrinks.

What about not pressure treated? Does it shrink so that one can see
in? I'm pretty sure these pickets aren't PT, (and I know mine aren't,
but my 42" fence isn't stockade. It alternates between pickets and
open spaces).

I suspect that unless the wood is marked "kiln dried" it's gonna shrink.
That's what wood does - some woods more than others, of course.
Nevertheless, the drier the wood, the less it will shrink.

Of course when dry wood gets wet - like being rained on - it will swell. In
a fence, however, that won't matter since the picket is trapped between
neighboring pickets.