2.4 is really old, originally released in 2001. 2.6 has also been around for a long time, and the release cycle changed during the 2.6 timeframe, so the old release number system no longer makes sense.
Rather than having a major stable release like 2.4, and then a long period of development in 2.5, followed by a stable 2.6 kernel later on, it switched to much shorter merge windows and release candidate cycles that last about 2-3 months. So you had 2.6.x kernels which were new stable branches, and would get patches such as 2.6.x.y.
After a while, this got a little silly. When numbers were getting up to things like 2.6.32.60, it gets hard to keep track of the numbers. Since the first two numbers were just sitting there unused, at some point Linus decided it was silly, and they would cycle to 3, and the second number would be for each stable release.
So yeah, 2.2-2.4 was about 2 years between new stable versions. 2.4-2.6 was about 3 years between new stable versions. But then there have been new stable versions about once every 2-3 months, and new long term supported versions picked from those about once a year (for distributions which prefer to do long-term support on a single kernel version, rather than updating to the latest stable).
> After a while, this got a little silly. When numbers were getting up to things like 2.6.32.60, it gets hard to keep track of the numbers. Since the first two numbers were just sitting there unused, at some point Linus decided it was silly, and they would cycle to 3, and the second number would be for each stable release.
And now we have only one number sitting there unused! Maybe they'll finish the job next time.
Rather than having a major stable release like 2.4, and then a long period of development in 2.5, followed by a stable 2.6 kernel later on, it switched to much shorter merge windows and release candidate cycles that last about 2-3 months. So you had 2.6.x kernels which were new stable branches, and would get patches such as 2.6.x.y.
After a while, this got a little silly. When numbers were getting up to things like 2.6.32.60, it gets hard to keep track of the numbers. Since the first two numbers were just sitting there unused, at some point Linus decided it was silly, and they would cycle to 3, and the second number would be for each stable release.
So yeah, 2.2-2.4 was about 2 years between new stable versions. 2.4-2.6 was about 3 years between new stable versions. But then there have been new stable versions about once every 2-3 months, and new long term supported versions picked from those about once a year (for distributions which prefer to do long-term support on a single kernel version, rather than updating to the latest stable).