I have the vertices selected as shown. I select “Mesh->Faces->Convert Triangles to Quads” but nothing happens. What am I doing wrong? I am using ver 2.42a.
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Just select the two triangles and press F or Alt + J.
I think you did it wrong.
What you need to do is press space bar > Edit > Faces > Convert to Quads.
You DON’T go Mesh->Faces->Convert Triangles to Quads because you might think it says Convert Triangles to Quads but infact, it only says Convert to Triangles not Convert Triangles to Quads.
This happens all the time to me, I don’t think your doing anything wrong.
Edit: To reproduce this (bug?), make a cube extrude a face and scale it down (quite small or you’ll end up with quads), now AKEY, CTRL+T, ALT+J & you’ll end you with this…
Thanks Wolf. I did try your suggestion without any luck. I’m also attaching a screen shot to show you that it does indeed say “Convert Triangles to Quads.”
I’m going to start over. The only thing I can think of is that I messed up the mesh so bad that for some reason it’s just not working.
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haha to slow 
Hey, there’s no need for that. just select and delete the middle edge, select the four verts & hit the FKEY.
Sorry N30N, I didn’t see your previous post.
Rant: I don’t know about anyone else but I think this new Forum Theme stinks!!! (IMHO)
N E Way… you seem to be correct as it does appear to be a “bug” in 2.42. I downloaded the November CVS build and it works just fine now.
Thanks for the help guys! Much appreciated!
I think that if the two triangles are not “flat” with respect to each other, Blender cannot convert the two triangles into a quad.
Imagine two paper triangles lying on a table edge to edge. Blender could convert those two triangles into a quad. Now, imagine that the edge where they are connected is a hinge. If the hinge is bent, the two triangles are not “flat” with respect to each other, and Blender can not convert those into a quad.
A general note on tri2quad convertion … i think it’s safe to say that using the script under Mesh -> Scripts -> Triangles to Quads produces results that are a lot better than the “Convert to Quads” function and the meshes are very clean after using it (inclduing edge-loops).
The “Convert to Quads” function even merged two triangles that where seperated by an edge of the object (i.e angle > 45°) some times 
Werner
The convert to quads funcion (alt+J) has been rewritten in current CVS version to match the triangles to quads script functionality but faster. Current alt+J in 2.42a uses the old method, which is crappy for several quads, but should work for 1 quad. Maybe you need to put the central vertex further so it has the shape of a quad; then, after alt+J, put it where it was before. Using snap from/to cursor function can help (shift S).
When I do organic modelling (which doesn’t seem to be your case) and I can’t convert to quad 2 triangles, I smooth it several times (W smooth) until it looks like a quad.