Box section or twin wall rims.

[blondini]

What defines a box section or twin wall rim and do these terms mean one and the same?

I've taken delivery of some 20" BMX wheels for a 'project' and although the ebay listing gave descriptions including the words 'Box section rim' and 'Alloy double walled rim for increased strength' they appear to be single wall to me. There may be small hollow section strengtheners at the corners of the shoulders but there's just a single wall across the centre.