Definition
A threaded fastener with a rounded, smooth head and a square shoulder directly beneath the head. The square shoulder seats into a matching square hole or recess in the material being bolted, which prevents the bolt from rotating while the nut is tightened from the opposite side.
Plain English
A bolt with a smooth domed head and a square section just under the head. The square part grips the hole so the bolt holds still on its own while you tighten the nut on the other end.
Context Anchor
Seen in aircraft maintenance and hardware descriptions, especially where a bolt passes through wood or other soft material.
Derivation
Named after the carriages (horse-drawn wagons and early coaches) where this bolt was originally used. The smooth rounded head sat flush against wooden bodywork without snagging, and the square shoulder locked it into the wood so a single worker could tighten the nut from underneath.
Why Pilots Care
Carriage bolts are not generally approved for primary aircraft structure, but pilots and owners doing maintenance work, building hangar fixtures, or reading parts catalogs need to recognize the type and know it is a hardware-store fastener, not an aircraft-grade bolt like an AN or MS bolt.
Intuition Check
Do not read “carriage” as meaning the carrying of people or cargo. Here it names a specific bolt shape originally used in wooden carriage construction.
Example Sentence 1
The mechanic chose a carriage bolt for the wooden workbench because the square shoulder kept the bolt from spinning while he tightened the nut.
Example Sentence 2
During fairing installation, carriage bolts held the panels securely while keeping the outer surface smooth.