Feature control frame
Every geometric tolerance on every drawing lives in one of these, and reading it fluently is the whole skill.
What it controls
The feature control frame — the tolerance indicator, in ISO 1101’s own wording — is the rectangle that carries a geometric tolerance. It is read left to right: which characteristic is controlled, how wide the zone is, and what it is measured against.
Nothing in it is decoration. Each cell changes what the callout demands, and a diameter symbol or a material modifier in the tolerance cell can change the requirement more than the number beside it does.
The tolerance zone
The frame states the zone rather than being one: the first cell gives its shape, the second its width, and the rest fix where it sits.
Try it in 3D
Select a cell to read what it contributes.
Characteristic
Which geometric control this is. It also tells you the shape of the tolerance zone and whether datums are needed at all.
The axis of this feature has to lie inside a cylindrical zone 0.1 mm across, positioned by the theoretically exact dimensions relative to A, then B, then C.
What else can appear in or beside the frame
- Diameter
- Makes the tolerance zone cylindrical instead of two parallel planes. Without it the value is the full width of a slab-shaped zone in the stated direction.
- Maximum material requirement
- After the value: the tolerance applies at maximum material size and grows as the feature departs from it. After a datum letter: the part may shift within the datum reference frame by the same reasoning.
- Least material requirement
- The mirror image, applied where the concern is wall thickness rather than assembly — a feature breaking out of a wall, or a boss becoming too thin.
- Envelope requirement
- On a size dimension, not in the frame. It links size and form: the feature has to stay inside a perfect-form boundary at maximum material size. Under ISO 8015 size and form are otherwise independent, so this is how that link is made.
- Projected tolerance zone
- Moves the zone OUT of the part, into the space a stud or dowel will occupy. Written with a height. Without it a tapped hole is judged only through the plate while the stud is free to lean outside it.
- Free state
- The tolerance applies to the part unrestrained. Used on thin, flexible or moulded parts where clamping changes the answer; the restrained case is stated with a note per ISO 10579-NR.
- Common zone
- Several separate features share one tolerance zone, held in a fixed relationship to each other. This is how ISO states what ASME draws as a composite frame.
- Unequally disposed zone
- A profile zone with more of the tolerance on one side of true form than the other, written with a signed offset. Useful where a surface may grow but not shrink.
Reading the callout
Position, to a cylindrical zone 0.1 mm across, located by theoretically exact dimensions relative to datums A, B and C in that order.
When to reach for it
- Reading a customer drawing whose geometric requirements decide whether a quotation is realistic — the frames are where the cost is.
- Checking whether a frame attaches to a surface or to a size dimension, since the same symbol demands different things in the two places.
- Meeting a mark inside a frame you cannot name — the list further down covers everything that can legally appear there.
How it is measured
- The frame tells the inspection how to proceed: the datum cells give the setup, and the tolerance cell gives what is being fitted to the measured points.
- A leader pointing to a surface controls that surface; a frame attached to a size dimension controls the derived axis or median plane instead.
- Modifiers change the method — a material condition modifier often means a functional gauge can accept the part in one operation.
Getting it right
- Read where the frame attaches as carefully as what it contains, since the same callout means one thing on a surface and another on a size dimension.
- Treat the datum letters as an ordered list, because their sequence in the frame is what makes one primary and another secondary.
- Look for the diameter symbol in the tolerance cell, since it turns a slab-shaped zone into a cylindrical one and changes the allowed error by a useful amount.
Common questions
What order are the cells read in?
Left to right, always. The first cell is the geometric characteristic, the second is the tolerance value with any modifiers, and the cells after that are the datum references in order of precedence. There is no version of the notation where a later cell outranks an earlier one.
What does a diameter symbol inside the frame mean?
It makes the tolerance zone cylindrical. Without it, a position tolerance of 0.1 mm gives a zone 0.1 mm wide in the stated direction; with it, the zone is a cylinder 0.1 mm across, which the axis can wander inside in any direction. The cylindrical form matches how round features actually assemble, so it is the common case for holes.
Does it matter where the frame points?
Yes, and this is the most common misreading. A frame with a leader touching a surface controls that surface. A frame placed under or beside a size dimension controls the feature derived from that size — the axis of a hole or the median plane of a slot. The symbol can be identical; the requirement is not.
Can one feature carry more than one frame?
Yes. Stacked frames are common and are read as separate requirements that all apply: a hole might carry a position tolerance to three datums and, below it, a tighter perpendicularity to the primary datum alone. Each frame is satisfied on its own terms, and the tighter one governs wherever they overlap. One case reads differently and is worth knowing on sight: where a single characteristic symbol spans two rows of one frame, that is a composite tolerance, and the lower segment refines orientation and the spacing within a pattern rather than restating the location to the datums.
These pages explain the published standards for reference and describe the tolerance zones they define. The values shown in the examples are illustrative. What a given part can hold depends on its geometry, material and process, and is agreed per drawing.