Revision history blocks, explained

Under ASME Y14.35 a drawing's revisions run through twenty single letters:

1. A
2. B
3. C
4. D
5. E
6. F
7. G
8. H
9. J
10. K
11. L
12. M
13. N
14. P
15. R
16. T
17. U
18. V
19. W
20. Y

After Y come AA, AB and so on to AY, then BA to BY, skipping the same letters. A revision is never more than two characters.

The first release needs no letter. It may show a dash in the REV box, and A is then the first change after release. Some companies release at A instead, so the first row of the revision block shows which scheme a drawing follows.

Not every drawing follows the rule. Older drawings, and some companies' own schemes, use the skipped letters (I, O, Q, S, X, Z), so a revision S is not necessarily a misprint. Others number their revisions, 1, 2, 3 or 01, 02.

• A revision symbol on the drawing: the revision letter, in a small symbol such as a triangle or a circle, placed beside the changed feature.
• The description, written so the change can be found.
• The zone in the ZONE column: the grid square from the letters up the sheet's sides and the numbers along its top and bottom.
• The change document's number, in the description or a column of its own.

Zones are read like a map's grid: B3 is the square in row B and column 3. How a sheet's zones are laid out is on drawing sheet sizes.

• ECR, engineering change request: asks for a change.
• ECO, engineering change order: usually authorizes the change and directs the work.
• ECN, engineering change notice: usually records the change and tells those affected.
• EO, engineering order, and ECP, engineering change proposal, the name government contracts use for a proposed change.

Companies use these names differently, and many use only one of them. What matters on the drawing is the number: it ties the revision to the paperwork that says why it was made. All the changes one change document authorizes go into the drawing at the same time, under one revision letter.

1. The REV box in the title block states the drawing's revision.
2. The revision block's latest entry, usually its last row, should carry the same letter. If they disagree, the drawing contradicts itself, and that is worth raising before anything is made from it.
3. On a drawing of several sheets, check whether each sheet has its own letter: ASME Y14.35 allows revisions to be kept for the drawing as a whole, sheet by sheet, or with every sheet at the same revision.
4. A print is a snapshot. It shows the revision it was printed at. Whether that is still current is answered by the company's document control, not by the paper.

Redrawn and replaced drawings. A drawing redrawn with changes moves to the next letter; one redrawn without change, to replace a lost or worn original, need not. A drawing replaced by one with a different number says so in its revision block, naming the drawing that replaces it.

Dash numbers let one drawing define several variants, such as a part in three lengths: each is the drawing number with a suffix, 10045-1, 10045-2, 10045-3, and each is a part number of its own. A drawing that lists its variants in a table this way is a tabulated drawing.

A revision belongs to the drawing, so every dash number on it shares the drawing's revision, and the revision is not part of the part number: under ASME Y14.100 a part number never includes the drawing's revision. Part 10045-2 is ordered as 10045-2; the revision says which version of the drawing defines it.

When a change needs a new part number. If a change leaves the part interchangeable with the old one, in form, fit and function, a new revision is enough. If it does not, ASME Y14.100 calls for a new part number, and the assemblies that use the part change part number too, up to the level where interchangeability is restored.