A double coupler is a type of scaffolding coupler that is used to join two scaffold tubes together in a parallel 90 degree configuration. 

The double coupler is typically made of steel and hot-dipped galvanized, which helps to prevent rust and corrosion. The size of the coupler is typically 48.3mm x 48.3mm, which is a common size for scaffold tubes.

To use the double coupler, the two scaffold tubes are inserted into the coupler, and then the coupler is tightened using a spanner. The double coupler creates a parallel connection between the scaffold tubes, allowing for the creation of horizontal scaffold structures such as scaffold platforms, guardrails, and walkways.