How was this deck/fence panel built?
Yep, Dado Blade, Multiple passes with single blade, Router, Chisel, Specialty Plane to name most of them. Board spacing can be drilled/driven into the posts or boards screws, nails, staples, blocks or pegs. Or, end extensions but usually full height end slats are screwed to the boards &amp; then the whole panel is slid in as 1-piece, a rotated stair balustrade. Or, the boards can just have the centers cut out to leave end legs.