An Interactive Field Guide
True Position
Two shops, opposite coasts, the same drawing. They build two parts that look identical — and only one fits. Both shops are right.
Geometric Dimensioning & Tolerancing is the language engineers invented so a drawing has exactly one reading. It looks like a wall of cryptic glyphs. Here is the grammar — and every diagram on this site is drawn by code, not pasted in.
Read in order — but start at the beginning. Start with chapter i →
§Why the Language Exists
- i The Tyranny of Plus-or-Minus Why an entire second language had to be invented on top of ordinary tolerances.Start here →
§The Grammar
- ii Anatomy of a Specification The whole language fits inside one little boxed sentence — learn to read it left to right.read →
§Datums & Reference Frames
- iii Datums and the Datum Reference Frame Before you can say where, you must agree on from where — and that means nailing down all six degrees of freedom.read →
§The Symbols
- iv Form: The Controls That Need No Reference Four shapes a feature must hold all by itself, with no datum in sight.read →
- v Orientation: Angle Relative to a Datum Perpendicular, parallel, or at a stated angle — to something.read →
§Material Condition & Location
- vi Material Condition: MMC, LMC & Bonus How a tolerance can grow — for free — as a part departs from its tightest size.read →
- vii Position, Virtual Condition & the Gauge The workhorse of GD&T — where a feature lives, and the go-gauge that proves it fits.read →
§The Universal Control & Rotation
- viii Profile: The Universal Control One symbol that can stand in for almost every other — form, orientation, and location of a surface, at once.read →
- ix Runout: Controlling Rotation For things that spin — how much a surface wobbles as the part turns about its datum axis.read →
§Synthesis
- x Reading a Real Drawing Put it all together — walk a complete multi-FCF drawing and reconstruct the designer's intent.read →
§Where It's Going
- xi Model-Based Definition: The Drawing Dissolves Where it's going — GD&T moves off the 2-D page and onto the 3-D model itself, as machine-readable PMI.read →