Testing for older OS versions

Hi,

simple question, not so simple answers:

how do you test your project(s) to the previous OS versions ?

OS X: some test for 10.6, others: 10.7, 10.8; 10.9, 10.10
Windows: XP, Vista, 7, 8 (8.1) and eventually 10
Linux: the worst because it have potentially tons of different flavours, so you may have to make tests on far more versions than in the two previous OSes.

use a lot of virtual machines…

Jean-Yves:
on the same computer I suppose.

Thanks for your answer.

Nota: I had a lot of troubles (with the OS too) when I skip from XP (VirtualBox on Mavericks) to a true PC with Windows 8.1.

try the win 10 preview in VirtualBox, thus for for me everything was worked going back over from Win10 to, 8.1 and 7, my primary PC is Win7 but while I’m away from it I use Win 10
Vista and XP I don’t really cater for, if it works its a bonus lol