From the several sources I've completed this table:
| UBUNTU VERSION | UBUNTU CODE NAME | UBUNTU RELEASE DATE | UBUNTU LTS | DEBIAN CODE NAME | DEBIAN VERSION |
|----------------|------------------|---------------------|------------|------------------|----------------|
| 16.04 | Xenial Xerus | APRIL | YES | stretch / sid | 9 |
| 15.10 | Wily Werewolf | OCTOBER | | jessie / sid | 8 |
| 15.04 | Vivid Vervet | APRIL | | jessie / sid | 8 |
| 14.10 | Utopic Unicorn | OCTOBER | | jessie / sid | 8 |
| 14.04 | Trusty Tahr | APRIL | YES | jessie / sid | 8 |
| 13.10 | Saucy Salamander | OCTOBER | | wheezy / sid | 7 |
| 13.04 | Raring Ringtail | APRIL | | wheezy / sid | 7 |
| 12.10 | Quantal Quetzal | OCTOBER | | wheezy / sid | 7 |
| 12.04 | Precise Pangolin | APRIL | YES | wheezy / sid | 7 |
| 11.10 | Oneiric Ocelot | OCTOBER | | wheezy / sid | 7 |
| 11.04 | Natty Narwhal | APRIL | | squeeze / sid | 6 |
| 10.10 | Maverick Meerkat | OCTOBER | | squeeze / sid | 6 |
| 10.04 | Lucid Lynx | APRIL | YES | squeeze / sid | 6 |
Following page said:
Debian is under continual development. The latest release is Debian
8.4. It is also (currently) known as stable or by its codename "Jessie".
Following page said:
Debian Unstable (also known as sid) is one of the 3 distributions that
Debian provides (along with Stable and Testing).
According to mentioned posts and the above table one question comes to my mind: Is next LTS versions of Ubuntu based on actual testing version of Debian? If yes, then there is one exception that misleads me; Ubuntu 12.04 was based on Debian 7. As you can see Ubuntu 11.10 was also based on Debian 7 so Debian 7 was not testing release in that time but rather it was stable, or am I wrong? I've also seen that is not recommended to install Debian packages to Ubuntu and vice versa. Why is this problem and why it is not recommended to install packages for next Ubuntu LTS (Xenial Xerus) from current Debian testing (stretch)?
Best Answer
Up until Ubuntu 12.04, Ubuntu LTS releases were based on Debian testing; non-LTS releases have always been based on Debian unstable, and LTS releases too since the introduction of proposed migrations.
Ubuntu releases go through a number of phases during their development; up until the Debian import freeze, packages updated in Debian unstable (or added to Debian unstable) are automatically synced over to Ubuntu, with a few exceptions. It's actually common to upload a package to Debian unstable and see it synced to Ubuntu's development release before it migrates to Debian testing! Since Debian unstable is ever-changing, it doesn't really mean much to say that Ubuntu is based on a given version of Debian unstable, because there is no such thing. A given release of Ubuntu is based on Debian unstable as it was at the time of the Debian import freeze.
All supported releases of Debian and Ubuntu get upgrades after their release, as you would expect for security support. These will naturally tend to diverge since the aim is to keep the changes to a minimum in each release "branch" (Debian 7 and LTS, Debian 8, the various supported releases of Ubuntu).