Open-riser stairs look light and cost less lumber, but the model residential code limits the opening between treads so a 4 in sphere cannot pass. The wall framing takeoff laid out open-riser stairs without checking that.
It now does. A 9 ft (108 in) rise takes 14 risers of 7-5/7 in. With 1-1/2 in treads the vertical gap is 7-5/7 less 1-1/2, which is 6-3/16 in, so the layout adds a note: add risers (solid or slats) or use a thicker tread. A tread 3-3/4 in thick, or a typed limit of your own, clears it. Some codes relax the rule for stairs rising 30 in or less, and the note says so.
Limits of the check: it is the vertical gap, a slightly conservative stand-in for a true sphere test on a sloped stair, and it is tested on hand arithmetic only, not against a built stair or your local code edition. The nosing note, which belongs to solid risers, no longer fires on open ones.