Why are the *.04 releases supported for three years and the *.10 releases aren't?

I've been wondering why canonical releases a *.04 release (ubuntu), advertises that it is supported for three years, and then a *.10 release comes out and isn't supported for three years.

My question: what's different about the *.10 release?

Canonical has Long Term Support releases that are supported for three years on the desktop and 5 years on servers. These are released every two years on even numbered April releases. (So your comment about April releases is only half right. For example, 9.04 wasn't an LTS release but 10.04 was).

Canonical does this to ensure that people will have support (security fixes, backported drivers, etc) of their operating system for a long period of time, while enabling Canonical to have major releases on a comparatively fast and regular schedule. (There aren't many other operating systems that put out releases every six months).

It's great for server admins as well as large adopters of the system because they don't need to upgrade as frequently and they have a long term upgrade schedule ahead of time.

Note: from 12.04LTS, the LTS-versions are supported for 5 years

More info:

• What's the difference between a Long Term Support Release and a Normal Release?

That's not how it is, actually. Ubuntu is released every six months, approximately. This is among other things in order to stay current with Gnome, who also releases every six months. Other distros and upstreams are doing the same thing now, and I think the main argument is that it keeps the energy up on the developer side, and keeps the expectations high on the consumer side. Although most of the releases have been on time, the first LTS version, the Dapper Drake was delayed two months because it wasn't ready. So it became 6.06 instead of 6.04. We also had a 10.07 which was for ARM SmartBooks. That's one of the reasons why we use these nicknames. They're alphabetical, so if we're on Natty now, it makes sense that the next one is Oneiric, and that 12.04 will be called P-something.

Anyway. Every fourth version is marked as an LTS. This means that in even years, the april version is an LTS, but not in odd years. Why every fourth? Supporting an operating system is lots of work and you really have to limit it. But you also want to support some versions for a longer period of time so that businesses and normal consumers can use it without having to upgrade all the time. I suppose every two years is a nice and round number. Other than that, there's probably no good reason.

Note: from 12.04LTS, the LTS-versions are supported for 5 years