Definition
Thin solid lines on a drawing that extend outward from the edges, surfaces, or features of an object to indicate the exact points between which a dimension is measured. Extension lines do not touch the object itself; a small visible gap is left between the object and the start of the extension line.
Plain English
Skinny lines that reach out from the edges of a drawn part to show where a measurement starts and stops.
Context Anchor
Seen when reading aircraft maintenance drawings, part sketches, repair diagrams, or blueprint-style illustrations that give sizes and distances.
Derivation
From 'extension' (Latin extendere, 'to stretch out'). The lines literally stretch out from the object so the dimension can be marked clearly without writing on top of the part itself.
Why Pilots Care
Maintenance technicians rely on extension lines to read dimensions accurately when inspecting, repairing, or fabricating aircraft parts. Misreading which feature a dimension applies to can lead to incorrect rework or out-of-tolerance parts.
Intuition Check
Do not treat extension lines as extra part edges or cut marks. They are guide lines that show the limits of a measurement.
Example Sentence 1
The technician traced the extension lines on the drawing to confirm the bolt hole spacing before drilling.
Example Sentence 2
Extension lines must maintain a small gap from the part outline so the drawing remains easy to read during inspection.