Sounds like a simple command, but it doesn’t seem to be working for me (2.49a). I select 2 triangles, hit join and nothing happens. What might I be doing wrong?
I imported this mesh from a .x file, so it’s triangulated.
Are you using Alt + J? Or try menu, Mesh / Faces / Convert Triangles to Quads under Edit Mode.
Or select both triangles and press F
Richard
I was actually using the Join Triangles button in Mesh Tools panel (buttons window), but I tried Alt+J and I also tried F but they still seem to be triangles. I tried the menu option too.
If you attach your blend we can see what the problem is.
Richard
Select all vertices (A) and remove doubles (W 6). All the double vertices mean you are not selecting the faces you think you are.
Richard
Oh wow I didn’t realise there were doubles in there! Thanks for that, it works now 
Could someone tell me where this command is hidden in 2.5? I can’t find it…
What command, remove doubles? I believe it is in the same place, press W in edit mode and select from the popup list. For other face options also try Ctrl+F. If you can’t fne what you want try a search (Space)
Richard
I meant “join triangles”. Thanks anyway, I found it in ctrl+f where it’s called “tris to quads” now. Makes sense!