UPDATED: Shortly after we published this article, The Telegraph announced its plans to introduce a paywall for UK readers. It is offering two packages, one costing £1.99 per month giving access via the web and through smartphone apps, the second, costing £9.99 per month providing tablet app access and loyalty club membership.
The move marks a major shift, with two of the UK's four mainstream broadsheet brands now charging for online access. The Telegraph has adopted a metered model, providing free access to 20 articles a month before asking readers to pay.
The tide of gated access and paywall strategies is getting stronger. Extracing revenue from readers has gone from a niche activity among majoratively free publishers, to an every day tactic.
Both the Washington Post and the San Francisco Chronicle have announced plans to charge for online access, joining the country's two largest titles, the Wall Street Journal and New York Times. According to Pew Research Center nearly a third of US daily newspapers now charge for digital access in some form.
In the UK, both The Sun and The Telegraph are rumoured to be on the verge of following the FT and Times in launching paywalls (The Telegraph already charges users overseas; Sun proprietor Rupert Murdoch has signalled that all News Corp titles will be paid-for online).
Even stalwarts of the free model, including The Guardian, are charging a decent subscription fee for tablet and smartphone apps.
The many options available means even well established paywalls such as the FT's are constantly being refined. There are no hard and fast rules and the newspaper industry has no equivalent of a Sun Tzu-style Art of Gated Access Models - so here's our breakdown of the different models available.
1. The absolute paywall
Notionally the simplest idea available - absolute paywalls are nevertheless relatively rare among newspapers, although more common among other sectors such as B2B publications.
The reasoning behind the approach goes something along the lines of "all our content is worth paying for, so people should pay if they want to read any of it".
The obvious example of this is The Times. Consumers have to pay a minimum of £2 per week to access The Times or Sunday Times on the web - rising to £4 for access to smartphone or tablet apps.
Previously Google and other search engines from listing excerpts of articles in search results, though this was changed in September. The Times has also dropped its paywall completely for special events such as during the Royal Jubilee and it often lowers the drawbridge for campaigning articles and for specific events.
2. Content restrictions
Perhaps the most logical approach is to restrict some content to paying subscribers while leaving other content available to all. This approach is all about deciding which content is most effective as a marketing tool, and which content consumers will actually be prepared to pay for.
Different publications take very different approaches to this one. For instance, the FT's AlphaVille blog is hugely influential and provides a powerful promotional tool. Along with several other of the paper's blogs, it remains free for all. The FT tried to move some of these blogs inside the walled garden, but quickly reversed the decision.
In contrast, the San Francisco Chronicle plans to charge only for "premium content" including columnists, opinion pieces and arts and leisure content. News stories will still be free, presumably because that's the stuff people could easily get elsewhere.
3. Device-specific models
A very different lever employed by many newspapers is charging based on the method of consumption - attributing value to convenience and form rather than content.
Most paywalled newspapers charge more for access on tablets or smartphones than they do for simple web access through a browser.
One of the most interesting aspects of this is that the owners of Apple products - iPads and iPhones - seem more willing to pay for content. The Guardian charges both for its iPad app and its iPhone app (though the latter costs an almost charity-like £4 for six months' access). However, it doesn't charge for apps on Android devices.
Metering has become a key part of paywall strategies for most newspapers - a way to balance the need to put your content in front of as many readers as possible while persuading those who like it to get out their credit card.
NYT readers were initially allowed access to 20 articles per month for free - though that figure has since been reduced to 10. The paper has also been steadily closing off loopholes which have allowed resourceful readers to get around the limit.
5. First click free and sharing
Another challenge newspaper paywalls face is how to ensure the paywall doesn't stop consumers from serendipitously discovering articles through search, or being directed to an article via social media. Both routes are key to gaining new subscribers but even metered paywalls can get in the way of a consumer seeing an article that might be especially relevant.
Some publications offer "first-click free" (here's Google's explanation) allowing someone to follow one link from social media or search result to an article which they can read, but restricting subsequent navigation through a publication's website. The NYT paywall allows readers from social media regardless of hom many articles they've read that month.
There are various ways of tweaking the process, and for instance the FT allows non-subscribers to read articles found through Google web searches but not from Google News results.
-- Digital distribution gives newspapers many ways of pushing and pulling users to pay for content - but finding the right combination for a particular publication is a complex process that even titles with successful digital subscription businesses will be refining and tweaking for years if not forever.
Image via Flickr curtousey of ingo.ronner