The discipline designator
The letter at the front of a sheet number says whose drawing it is. G is general, C is civil, A is architectural, S is structural, M is mechanical, P is plumbing, E is electrical, and larger projects add more: F for fire protection, T for telecommunications, L for landscape, Q for equipment, I for interiors. On this set you will meet G, C, A, S, M, P and E.
That letter answers most location questions before you have thought about them. How big is a footing? Structural, so an S sheet. What size is the duct over the corridor? Mechanical, so an M sheet. How far is the building from the property line? Civil, so a C sheet. The letter is telling you whose scope the answer lives in, and scope is what determines who drew it.
It follows that a question can be asked of the wrong discipline. Asking an architectural plan for a beam size, or a mechanical plan for a wall dimension, will produce either nothing or a screened background that is not the record for that information. Knowing which discipline owns an answer is the first half of finding it.
- G, general: cover, index, symbols, abbreviations, general notes, code information.
- C, civil: site, grading, drainage, paving, site utilities.
- A, architectural: plans, elevations, sections, details, schedules, interiors.
- S, structural: foundations, framing, structural sections and details.
- M, P, E: mechanical, plumbing and electrical, each with plans, details and schedules.
The sheet type digit
The first digit after the letter says what kind of drawing it is, and the sequence runs roughly in order of how close you are standing to the building. Zero is general information for that discipline. One is plans. Two is elevations. Three is sections. Four is large scale views such as enlarged plans. Five is details. Six is schedules and diagrams. Seven and above are used for user defined and three dimensional content on sets that need them.
So A-201 is an architectural elevation, S-101 is a structural plan, M-101 is a mechanical plan, and A-601 is an architectural schedule sheet, and you knew all of that without opening any of them. That is the entire value of the convention: the number is a query rather than a name.
The digits after the type digit are a sequence within that type and carry no meaning of their own beyond order. A-101 and A-102 are the first and second architectural plan sheets, which on a two storey building normally means Level 1 and Level 2, but nothing in the numbering guarantees that. The sheet title is what confirms it.
Why the order of a set is what it is
A set is ordered by discipline and then by sheet type, which produces a sequence that moves from the general to the particular and from the ground up. General sheets come first because they define the vocabulary the rest of the set uses. Civil comes next because the site precedes the building. Architectural sits in the middle because it is the largest and most referenced group. Structural, then mechanical, plumbing and electrical follow, each drawing on the architectural background rather than the other way round.
Within a discipline, plans come before elevations, elevations before sections, sections before details. That is the same movement at a smaller scale: place it, then look at it, then cut it, then magnify one joint of it. Once you have internalised the ordering you can find an unfamiliar sheet in a set you have never seen, which is a genuinely useful skill on a large project.
The order is also why a sheet added late gets a number that fits the pattern rather than a number appended at the end. An enlarged plan added after issue becomes another sheet in the four hundreds, not a sheet at the back, because the number is an address and an address has to be in the right street.
The sheet index is the authority
The cover sheet carries an index listing every sheet in the set with its number and title. That list, not the thickness of the stack, is what tells you whether a set is complete. A set that never had a reflected ceiling plan is a different problem from a set that had one and lost it, and only the index can tell you which you are looking at.
The index is also where a late addition is recorded. When a set is revised and a sheet is added or deleted, the index is reissued, so comparing the index of the set you are holding with the index of the set the office is holding is the fastest way to find out whether you are working from current information.
Read the index once at the start of a project. It takes a few minutes and it replaces a great deal of flipping later, because it gives you the shape of the whole document before you need any particular part of it.
Reading a set you have never seen
The practical routine is short. Open the cover, read the index, and note which disciplines are present and how many sheets each has, because that tells you how complex the project is in each direction. Read the general sheets, especially the symbols legend and the abbreviations, since those define how the rest of the set speaks. Then go to the plan for the level you care about and work outward through its callouts.
Where a project is large enough to need more than one sheet for a single plan, look for match lines and for a key plan in the title block area, a small diagram of the whole floor with the part covered by this sheet shaded. The key plan is the fastest way to work out where you are on a big building, and it is often the only thing on the sheet that tells you.
Finally, note the issue status on the cover. A bid set, a permit set and a construction set are different documents with different purposes, and the difference is stated plainly on the sheet rather than implied.