GD&T fundamentals explained accurately: Rule #1 and Rule #2, what RMB means for a datum reference, a grounded bonus-tolerance example, and the 14 GD&T symbols grouped by what they control.
GD&T rewards precision, so it’s worth being precise about the standard
itself: “RMB” isn’t an old, obsolete term for something else — it’s the
current ASME Y14.5-2009 language, introduced specifically to clear up
confusion that MMC/LMC/RFS used to cause when applied to datums. If a
drawing note or a class you took uses different vocabulary than another,
that’s often just an edition difference, not a disagreement about the
actual rule.
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language,
standardized under ASME Y14.5, for specifying how much a part’s actual
form, size, orientation, and location are allowed to vary from the
drawing — and for defining that variation relative to real, measurable
reference features (datums) instead of vague notes. The point isn’t
precision for its own sake; it’s making the drawing say exactly what the
designer needs to be true for the part to function, no more and no less.
These two rules get referenced constantly and explained badly just as
often. Both are foundational defaults baked into every GD&T drawing,
whether or not they’re written out on it.
| Rule | What it means |
|---|---|
| Rule #1 (Envelope Principle) | For a regular feature of size, the feature’s surface can’t violate its Maximum Material Condition (MMC) boundary — the size tolerance itself limits form, unless a note like “independency” overrides it |
| Rule #2 | Where no modifier appears in a feature control frame, individual geometric tolerances default to Regardless of Feature Size (RFS), and datum feature references default to Regardless of Material Boundary (RMB) — MMC/LMC (or MMB/LMB for datums) only apply when explicitly called out |
The MMB/LMB/RMB terms specifically apply to datum features and were
introduced in the 2009 revision of the standard to replace the older
MMC/LMC/RFS language in that context — because a datum referenced “at
MMC” was really engaging a fixed virtual-condition boundary, not the
literal size of the part, and the old wording confused people about that
distinction. If you’re reading an older drawing or an older textbook and
see “datum at MMC,” it means essentially what a current drawing calls MMB.
This isn’t just terminology — it changes how a part actually gets
checked. An RMB datum reference means the fixture or gauge has to conform
to the datum feature’s actual, as-produced size — no bonus tolerance, no
shift allowed. An MMB reference, by contrast, lets the part shift within a
fixed virtual-condition boundary, which is what creates “bonus” tolerance
on the related feature. Setting up a CMM or a gauge without knowing which
one you’re working against is a common way a part fails inspection for a
reason that has nothing to do with whether it’s actually good.
Say a bracket has a position tolerance of 0.010″ on a bolt hole, called
out at MMC relative to a datum. If the hole is machined exactly at MMC
(its smallest allowed diameter), the position tolerance is exactly
0.010″ — no more. But if the hole comes out larger than MMC, closer to
its Least Material Condition, the position tolerance effectively grows: a
larger hole has more room to be slightly off-position and still let the
bolt pass through, so the standard allows a matching amount of “bonus”
tolerance.
That bonus tolerance isn’t a loophole — it reflects real assembly
physics, which is exactly why MMC callouts exist. Inspecting the part
without accounting for it means rejecting parts that actually function
fine, which is a real cost with no real benefit. This is also where the
RFS/RMB default from Rule #2 matters: without an explicit MMC or LMC
modifier on the drawing, none of this bonus tolerance applies at all —
the position tolerance stays fixed regardless of the hole’s actual size.
| Category | What it controls |
|---|---|
| Form | Straightness, flatness, circularity, cylindricity — shape of a feature independent of any datum |
| Orientation | Angularity, perpendicularity, parallelism — a feature’s angle relative to a datum |
| Location | Position, concentricity, symmetry — where a feature sits relative to datums |
| Profile | Profile of a line or surface — overall shape control, form and location combined |
| Runout | Circular and total runout — variation as a part rotates around a datum axis |
A few patterns show up repeatedly on the floor. Treating an unmodified
tolerance as if it were MMC by habit, when the drawing actually calls for
RFS by default under Rule #2, produces a part that fails inspection for no
real reason — or worse, one that’s accepted when it shouldn’t be. Confusing
a datum feature (the physical surface) with a datum (the theoretical
reference derived from it) leads to fixturing that doesn’t actually
simulate what the drawing intends. And reading an older drawing’s “at MMC”
datum note without recognizing it means the same thing as a current
drawing’s MMB modifier causes people to think the standard changed the
rule, when it only changed the label.
None of these are exotic edge cases — they’re the ordinary ways a
correctly-toleranced drawing turns into an incorrectly-inspected part.
Catching them is less about memorizing more symbols and more about
slowing down at the feature control frame before setting up a gauge or
a CMM program.
GD&T isn’t a standalone course in SCMEP’s current catalog — it’s the
language underneath our Inspection learning
path, feeding directly into the inspection criteria used in our
AS9100 and ISO 9001:2015 overview training. As a
NIST Manufacturing Extension Partnership affiliate
serving South Carolina manufacturers since 1989, our focus is
connecting GD&T literacy to real inspection planning — CMM programming,
gauge setup, first-article inspection — not just symbol memorization.
If your team can read the symbols but struggles to translate them into
an actual inspection plan, you can
browse the Quality and Inspection training
catalog or email the training team.
A fabricated part’s drawing often carries a second, separate symbol
system too — see our related article on
welding blueprint symbols for
how AWS A2.4 governs weld callouts, distinct from GD&T.
Reading a full blueprint takes more than GD&T fluency — title
blocks, revision history, and view types set the context GD&T callouts
live inside. See our related guide on
how to read a blueprint for
the first 5 things to check before you even start measuring.
GD&T callouts are what a CMM actually measures against. See our
related guide on CMM and
dimensional metrology for how coordinate measuring machines verify
the datum reference frames and tolerances covered here.
Rule #1 (the Envelope Principle) limits a feature’s form based on its size tolerance — the part’s surface can’t violate its MMC boundary. Rule #2 sets the default modifier: geometric tolerances default to RFS and datum references default to RMB unless a modifier is explicitly specified on the drawing.
RMB stands for Regardless of Material Boundary. It’s the current ASME Y14.5-2009 term for a datum feature reference that uses the feature’s actual size, with no bonus tolerance or shift allowed — it replaced the older “at RFS” language specifically for datum features.
MMC (Maximum Material Condition) and LMC (Least Material Condition) are size-based modifiers that allow bonus tolerance as a feature departs from that condition. RFS (Regardless of Feature Size) means no bonus tolerance applies — the tolerance is fixed no matter the feature’s actual size.
Yes — they remain the standard under ASME Y14.5 for specifying form, orientation, location, profile, and runout tolerances, and are still the expected language on engineering drawings across most manufacturing sectors.
