CSS: z-axis and IE

I have just had a problem with my menus suddenly disappearing behind a new div that I created.

It took longer than it should to fix the problem which is to do with a bug in the way that IE6/IE7 deal with the cumulative effects of declaring a z-pos for elements.

I found a long, illustrated, exposition of the problems called Effect of z-index value to positioned elements, which demonstrated the various issues, but only offered implicit solutions.

I found the solution that I actually needed at quirksmode.org. I probably would not have understood the significance of this if I had not read Aleksandar Vaci?’s article first.