A shackle is a piece of metal that has been bent into a U shape, where the opening of the shackle is secured by a clevis pin or bolt. Alternatively, a hinged loop may be used with a quick release pin if the shackle is required to be opened and closed on a regular basis.
D shackles are typically thinner than other shackles and are formed into a D or square/rectangular shape.
D shackles are more rigid in their structure than bow shackles and as a result are not used in situations where the orientation of the secured rope or cable may change e.g. a moored boat which may rock from side to side. D shackles are widely acknowledged to take greater loads however this comes at the detriment of it only being able to bear the load from one direction compared to multiple directions of a bow shackle.