How can they develop 2 continuous version of google chrome at the same time? [closed]

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I see some softwares are being developed more than one version at a time. Ex: Google chrome 15 is being developed when version 14 is still in Beta, Firefox 4 and 5 is being developed together. I wonder how could that be possible? What if there is a bug found in Chrome 14, does the team work on version 15 have to fix that bug again? Isn't working on just one version at a time would be more efficient and consistent?

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By using a source code control system such as Subversion, Git, Mercurial, etc. you can manage multiple branches of your code, and merge edits from one branch to another quite easily.

Branches are often used to maintain a current stable version of a product, in which you fix bugs but don't add major features, and a development version to which you add new features.

During the development of the new features branch, you periodically merge the bugfixes from the stable branch into it. The source control system automates that merge process to a large extent.