Microsoft Windows 7. Why number 7?

First we need to take a look at conventional software numbering. In traditional software developing first there is pre-alpha version, which is used for internal testing and may not include all features. Then there is an alpha version, that has all of the features and usuly is tested internaly by company testers. Then comes a beta version, that can be publicly downloaded and tested by anyone. When reported bugs has been fixed, the release candidate (RC) is released, and if no critical bugs are found, than it is the final release. Usualy there are many of release candidates. For example currently FlashDevelop is in it’s RC5 version.

Software numbering is as follows. All releases prior to 1.0 is either pre-alpha, alpha or beta releases. 1.0 is the first stable version of software, that is publicly available. All minor changes to software are indicated by increasing number after the decimal point. The major changes are indicated by changing the first number.

Now let’s look at Windows 7, and all the numbers of prior windows versions.
First there was Windows 1.0, then Windows 2.0, and than in year 1992 – Windows 3.0.
Windows NT has version number 3.1.
Now, Windows 95 was 4.0.1998, Windows 98 SE was 4.10.2222, Windows Me was 4.90.3000. So we’re counting all 9x versions as being 4.0.
Windows 2000 was completely new OS, since it has a completelty new architecture. The version number for Windows 2000 is 5.0. XP was similar to 2000, so it is version 5.1.
Windows Vista is again, completely new generation and thus deserves a new number Windows 6.0. There are number of software products working on windows XP, but not on Vista, because of architectural changes.
The windows 7, is not a completely new. It is a successor of Vista. Windows 7 was it’s code name. Similar as Longhorn was a code name for Vista. Designers simply liked number 7, since it is considered to be lucky. That is why the name is Windows 7. But from system view the number for this OS is 6.1. You can see Windows number by running cmd.exe.