On most structural sets I draft, we label grids A–H, J–N, P… and intentionally skip I and O to avoid 1/0 confusion at 1/8"=1’-0" on plotted sheets. Trivia question: does anyone know which standard first documented this (AIA, ASME, ISO?), or is it just a long-standing office convention that stuck because it reduces field misreads during erection?
I haven’t seen a single ‘first’ source — it’s a long-running convention that shows up in NCS/UDS and ISO 4157 as avoiding ambiguous grid letters (skip I and O), the no-13th-floor of drawings. If you need something cite‑able, point to the NCS at https://nationalcadstandard.org and add a tiny note like “grids omit I and O to ‘avoid 1/0 confusion’”; anyone have a clause number handy?
ISO 4157-1 spells it out — “I” and “O” shall not be used (some shops also drop “Q”), so it’s more than convention. From experience, I add a tiny grid legend on the cover (A–H, J–N, P…) so field crews know what’s skipped before layout. @sophia_g89 do you roll to AA after Z or switch to 10-series?
On a couple of steel frames at “1/8"=1’-0"”, I keep the I/O skip but also switch the plotted tag font so zeros get a slash and 1s have serifs — cheap way to kill misreads without changing grids. Caveat: some reviewers dislike slashed zeros on dims, so I limit that style to grid bubbles only. In Revit, do you let it roll to AA/AB past Z while preserving the skip, or cap at Z?
, same pain here at “1/8”=1’-0"": we skip I and O, and my one concrete step is a Revit preflight — Dynamo script that flags any grid name matching [IOio] and blocks the PDF set… I haven’t found an AIA or ASME “first citation,” just later adoptions; on the fab side they also ask us to avoid those letters on member marks to keep erection tags clean. Anyone have an AIA/UDS page number I can cite in our template note?
I upsize grid bubbles at “1/8”=1’-0"" and use squares for letters, circles for numbers; renovations get grandfathered. NCS mention anywhere?
Feels more like tradition than a standard — I haven’t seen a hard call in NCS or ISO, more like example sheets (kind of like skipping the 13th floor). My one tweak: put “Letters ‘I’ and ‘O’ are intentionally omitted from column lines” in the cover-sheet conventions and repeat it once on the first plan near the title block so field crews see it. @lgarcia50 got a clause number anywhere?
I haven’t seen a U.S. mandate; the only explicit callout I’ve run across is EU-side (DIN 1356-1/EN ISO 4157 avoids ambiguous letters), so I treat it as best-practice. My one tweak: I print a tiny legend on the first S plan with the exact run “A–H, J–N, P…” so shop folks don’t guess; @mconnor22, have you ever seen an NCS clause that spells it out?