Meet DIN603 specification.
A carriage bolt, coach bolt or round head square neck bolt is used through iron strengthening plates on either side of a wooden beam.
It is commonplace though to use them on bare timber, the squared section giving enough grip to prevent rotation.
Carriage bolts are extensively used in security fixings where the bolt must only be removable from one side.
The smooth domed head and square neck below prevent the carriage bolt from the insecure side.