Me and a friend are trying to build a trailer using principles found here ...

<http://www.n55.dk/MANUALS/SPACEFRAMEVEHICLES/DIY.pdf>

They are using 25 mm aluminum square tubing with a 2mm thickness and 6.5mm holes.

At the bottom of the docs is a bill of materials listing the materials.

It appears they are just drilling holes and than inserting a bolt and nut to connect the pieces.

Is this what they are doing?

More specifically does their need to be a washer on both sides?