What is the effect of the M modifier in a GD&T callout?

• A. It ties the tolerance to MMC; the tolerance zone is evaluated at the MMC boundary.
• B. It tightens all tolerances uniformly.
• C. It changes the projection height automatically.
• D. It applies only to straightness tolerances.

Answer: (A)

The important idea here is that the M modifier stands for Maximum Material Condition and it anchors a tolerance to that extreme size. When a tolerance callout includes M, the specified tolerance is interpreted at the feature’s MMC — the size at which the feature contains the maximum material (external features at their largest size; internal features at their smallest size). What this means in practice is that the tolerance zone is defined relative to the MMC boundary, and as the actual size moves away from MMC, the tolerance can effectively relax or “bonus” accordingly.

So, for a callout with M, you evaluate whether the feature meets the tolerance at the MMC size. If the feature size changes away from MMC, you don’t re-derive the tolerance from scratch; instead, the MMC relationship governs how the tolerance is applied, often allowing more variation than if MMC weren’t specified. That’s why it ties the tolerance to MMC and the tolerance zone is evaluated at the MMC boundary.

The other options don’t fit because this modifier doesn’t uniformly tighten all tolerances, doesn’t automatically change projection height, and isn’t limited to straightness—it applies to the tolerance in the context of MMC for the given feature.