It means each corner is made up of two rails meeting the post rather than one, with heavy-duty corner bracing bolted across the junction. This doubles the contact area at every corner and gives the frame significantly more resistance to racking — the twisting load that builds up when wind gets under a large structure or when a polycarbonate roof adds weight across the span.

Fifty-one sizes, from 1.8m deep by 1.8m wide up to 3m deep by 7.8m wide, stepping in 600mm increments. Depth is wall to outer post, width is how far the structure runs along the house. Bespoke footprints are available for dimensions between or beyond the listed range.

Yes, more so than the standard garden build. The double corner bracing is the specific reason people choose this model when a roof kit is going on. A polycarbonate roof adds wind loading to the frame, and the doubled corners and bracing are what keep the structure square as that load builds up over time.

A sound, load-bearing house wall — solid brick, blockwork and rendered block all work. The wall plate is secured first, then the outer posts are set in concrete or fixed with bolt-down feet. Wall fixings are included. The wall needs to run the full width of the pergola.

A little. The double rails and additional corner bracing components mean there are more pieces to fit at each corner. Two people with a drill and a level can still build most sizes in a day, but allow an extra few hours compared with the standard garden model on the larger footprints.