A timber picket is shaft and head cut from one board, not a shaft with a separate cap bolted on. A saw or router cuts the head profile straight into the flat face of the timber, so the whole picket is one piece from foot to tip. Standard Australian sections are 65x19, 70x19, and 88x19 DAR (dressed all round) treated pine, or the same sizes in hardwood for a heavier, longer-lasting fence. Pickets are face-fixed to the front of two or three horizontal rails with screws or nails, the same face-fixed-front convention a spear picks up its shaft with on a steel fence.
Five head styles cover the trade: pointed (also called Gothic), dog ear, round, French Gothic, and flat. Pointed and dog ear are the everyday picket tops on a suburban front fence. Round softens the look, French Gothic is the decorative option for a period property, and flat is the plain square cut with no shaped head at all.
Spacing is usually a fixed 50 mm face-to-face gap, which is what most people picture when they hear "picket fence." Set the gap to 0 mm and the pickets butt tight against each other, which is a paling fence rather than a picket fence, even though the boards and the fixing are identical. The picket foot sits clear of the ground, typically 50 mm, so the end grain stays out of standing water.
CAD60's timber picket family builds the picket as one polygon outline extruded straight up, so the head profile, the shaft, and every dimension line come from your entered head style and section in one pass.