Here's a great way to make a simple 90 degree corner with 25 x 25mm RHS. Perfect for making light weight gates or panels. Economical and easy to use.

Available individually or cheaper in a pack of 25

General Purpose Purlin Brackets, to suit: 

C100

C150

C200

Holes are punched to suit the standard end punching on the appropriate size. GP Brackets allow you to make a 90 degree joint from either the open side or the closed side of a C-Purlin

Pergola Brackets, as the name implies, are intended for use in light structures such as pergolas, but they come in handy for dozens of other uses as well.