Aluminium T-track bars have a channel that can be used to attach gear (such as roof boxes or bike carriers). This can be useful as you can use the full length of the roof bar.
When we talk about the size of the T-track channel we are referring to the width of the channel (shown by the green arrow in this diagram), rather than the opening on the top of the bar. You can usually remove an end cap to access this channel.
All the roof bars we sell have a standard size T-track channel that takes accessories with a 20mm T-bolt. This is by far the most common size of T-track channel and is used by Thule, Atera, Yakima/Whispbar, Cruz and Dropracks (as well as many roof bar manufacturers own-branded bars).
Some roof bars (notably BMW branded bars) have a wider T-track channel. If you want to use the T-track to attach gear to these bars you'll need to be sure that the supplied T-track fittings work with these bars. Alternative adapters are available for some products.
Some roof bars have a T-track channel that is smaller than 20mm. If your bars have a smaller T-track channel you will need to use fittings that go around the bars (e.g. U-bolts or clamps) as this narrower T-track isn't suitable for attaching gear to directly.