Paradigm shifts are often preceded by tiny, almost unnoticeable shivers. So you could be forgiven for missing the news that late last month, the government of the Commonwealth of Massachusetts (for historical reasons it does not call itself a state) decided that all the documents its employees create have to be in a data format called OpenDocument.

What makes this more than an obscure bit of United States government administrivia? Well, it could be the trigger for a revolution that will increase consumer choice and ensure the survival of documents that could be of historical importance in the future.

OpenDocument, as its name implies, is an "open" format. That is, one where the way in which your information is digitally represented in ones and zeroes is carefully specified, documented and made available without restriction. This means that anyone can create software that reads or writes it.

Closed formats, by contrast, are undocumented, or perhaps documented but with patent or other licensing restrictions on who can create software to manipulate them. The classic examples of closed formats are those of Microsoft Office - all Microsoft's recent formats have been undocumented (as far as we know). The new formats for the next version will be documented - but restricted.

The main problem with closed formats is known as "vendor lock-in". If all your data is locked up in files which only one piece of software can read, you are forced to buy that software for any new computers you get, whether it is entirely suitable for your needs or not.

This problem is compounded by what are called "network effects". If you are buying a car, you have great freedom, within the constraints of budget, to buy a car that suits your needs and sense of aesthetics. It doesn't matter what car your friends, family or customers have - they all run on the same roads. However, when exchanging documents, if your customers’ or friends’ software can't read the documents you send, you aren't communicating. This is the network effect - what you use affects what others use, and vice versa. Strong network effects in a marketplace with more than one vendor normally leads to market instability. The common long-term stable state in such a market is a monopoly.

What's more, network effects on a closed format put you on the upgrade escalator. When others upgrade, you need to if you want to continue reading the files they send you. And when you upgrade, those you communicate with have to upgrade in turn. The software vendor has no incentive to make the new formats completely "backwardly-compatible". After all, if they do, there is much less incentive for you to purchase the upgrade, and so they make less money.

The second problem with closed formats is obsolescence. It may seem unlikely now that there will ever be a time when it's hard to find something to read Word files - but where would you go today to find hardware to read a 5 1/4" floppy disk?

Governments such as Massachusetts are concerned about long-term data preservation. We need to care about it too - there would be no point in having the 30-year-rule, which lets us read the inside story on all the Government scandals of the 1970s, if all the data was stored electronically in a format no-one understood any more.

All these problems are left behind by open formats. Anyone can implement them. The people behind www.OpenOffice.org, www.koffice.org and www.abiword.com have all done so - and there is no vendor lock-in. The documented and free nature of the format means that obsolescence is not possible. As long as the format has users, there will be software to read and write it.

Open formats are an important part of computing freedom (although alone they are not sufficient) because they give people full control of their own data. In future years, when this freedom is commonplace, I predict that the Massachusetts Decision will be seen as the turning point.

Gervase Markham works for the Mozilla Foundation, a non-profit organisation dedicated to promoting choice and innovation on the internet. His blog is Hacking for Christ

Times Recommends

• Equitable Life halts £7bn sale on market turmoil
• Potential buyers emerge to save Woolworths staff
• Terrorist attacks close Bombay Stock Exchange