Thanks Andy, but that doesn't quite solve the problem. Or, it solves
one but creates another, because it spreads the width of the group by
inserted a middle column. Effectively, that XML creates 3 columns,
each 3 rows. What I want is 2 columns that are each 2 rows, with a
single row (but 2 columns wide) underneath in the third row.

You did get me thinking about boxes and I thought the code below would
solve the problem, but it doesn't. It still places the final button
into a third column. What this code attempts is to put the two
vertical boxes into a horizontal box, with a button below that (and
putting the button into a box does nothing, as you'd expect).

Any other suggestions?

Paul

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/
customui"
<ribbon
<tabs
<tab idMso="TabHome"
<group id="grp1" label="Test"

<box boxStyle="vertical" id="box1"
<box id="box2"
<box boxStyle="vertical" id="box3"
<button id="btn1" label="Top L"/
<button id="btn2" label="Mid L"/
</box

<box boxStyle="vertical" id="box4"
<button id="btn3" label="Top R"/
<button id="btn4" label="Mid R"/
</box
</box

<button id="btn5" label="Bottom"/
</box

</group
</tab
</tabs
</ribbon
</customUI