The rows, from the outside in
The outermost row is the overall dimension: one number for the whole building in that direction, normally taken grid to grid or face to face. Inside it sits the grid string, one number per bay, which is what the structural engineer and the layout crew work from. Inside that sits the locating string, which places every partition, opening, jamb and offset along the same run.
Each row is a finer sieve than the one outside it, and the numbers in a row add up to the number in the row outside. That redundancy is the whole point. A set with only a locating string would give you no quick way to check it; a set with only an overall would tell you nothing useful. Two or three rows in parallel mean any single mistake shows up as a sum that does not work.
Read them outside in, and you get the building at three resolutions in the order that makes each one intelligible. Read them inside out and you are adding fifteen numbers before you know what the answer should be near.
- Overall: the whole building in one direction, outermost row.
- Grid: bay by bay, centre line to centre line.
- Locating: to each partition, opening and offset, innermost row.
- Occasionally a fourth row for a specialist trade, such as a curtain wall module.
The anatomy of one dimension
A dimension is drawn as a small assembly of parts and each part means something. Two thin extension lines run out from the two points being measured, with a small gap where they leave the object so they cannot be mistaken for part of the building. A dimension line runs between them and is terminated at each end, by a slash tick on architectural drawings or an arrowhead where space is tight. The number sits on the dimension line, usually in a gap left in it.
The extension lines are the part to read carefully, because they tell you what is actually being measured. A number floating near a wall with nothing reaching real geometry is a note, not a dimension. A number whose extension lines land on the faces of a wall is measuring the wall; one whose extension lines land on the grid is measuring to the centre line and includes half the wall on each side.
Feet and inches are written with a plain hyphen between them, so twenty four feet exactly reads as a feet mark, a hyphen, a zero and an inch mark. A dimension in inches only, common on details, omits the feet entirely. Recognising the notation quickly matters more than it sounds, because a set can mix formats between disciplines.
What face is it taken to
The same wall can be dimensioned three different ways: to its centre line, to the face of the structural framing, or to the face of the finished surface. Those differ by half a stud and by the thickness of the board, which is enough to matter for a corridor width, a clear opening or a piece of equipment that has to fit.
A set states the rule once, in the general notes, rather than repeating it at every string. Dimensioning to the face of stud is common on framed construction because the layout crew snaps their lines on studs and the finishes go on afterward; dimensioning to grid or centre line is common on structural drawings because a centre line does not move when a member changes size. Read that note before reading any number, because it silently applies to all of them.
Where a particular dimension breaks the rule, it will say so, either with a note at that string or by the extension lines landing somewhere obviously different. Those exceptions are not sloppiness. They are usually the drafter insisting that one specific distance is more important than the convention.
CLR and EQ are instructions, not measurements
A dimension marked CLR states a clear distance that must be held. It is a requirement about the finished space rather than a description of where a line was drawn. If a wall assembly gets thicker between design and construction, the wall moves and the clear dimension stays, and everything else in the string absorbs the change. Corridors, accessible clearances and equipment access are the usual places to find one.
A dimension marked EQ says the drafter cared that a run be divided evenly rather than that it be a particular figure. Three bays marked EQ across a wall mean the builder measures the actual run and divides it into three, which produces a result that looks right even when the overall has moved by an inch. Writing a number instead would have produced a visible error the moment anything shifted.
Both marks are ways of saying which number is allowed to move. That is the deeper idea worth taking away: a dimension string is not a list of facts, it is a description of priorities. Reading it well means knowing which numbers are commitments and which are consequences.
- CLR: hold this clear distance whatever happens to the construction around it.
- EQ: divide the run evenly; the actual figure follows from the run.
- TYP: this dimension applies at every similar condition, not only where it is drawn.
- V.I.F.: verify in field, meaning the number depends on something that exists already.
- MIN or MAX: a limit rather than a target, and usually driven by a code requirement.
Missing numbers, and what to do about them
A carefully dimensioned plan is deliberately incomplete. A drafter leaves one segment of a string undimensioned when the rest of the string plus the overall already fixes it, because putting the number in would create two ways of arriving at the same distance and therefore a chance for them to disagree. If you find a run with exactly one gap, that is usually why, and you can solve for it.
If you find a run where every number is given and the sum does not match the overall, the drawing has contradicted itself. That is a defect, not a puzzle. Report it, and do not average the difference or favour the number that suits you, because somewhere downstream another trade is reading the same string and will resolve it differently.
If a dimension you need is simply absent and cannot be derived, ask. Scaling it off the paper feels like initiative and produces a wall in the wrong place, because the paper was never the record of intent. The numbers are, and a number nobody wrote is a decision nobody has made yet.