Though as Microsoft always maintained, they're not exactly phones, which meant Microsoft had a big marketing job on its hands.

It didn't make life easy for itself. The original Duo launched with flaky software, an outdated processor with no support for 5G or NFC, and a poor camera. "Don't compare it to a conventional phone" wailed Microsoft, but of course people did and given its ultra-premium price they found it wanting.

To be fair Microsoft put most of this right with its second generation Duo 2, but by then the damage was done.

Sadly, especially for Microsoft, but also for those of us who believed in the concept, the market has moved on to a different idea of what a folding phone should be.

Unlike the Duo series, the Samsung Fold, Pixel Fold and One Plus Open present themselves as conventional phones: when folded their front screens should function like any other phone (less so in the case of the Samsung, but it does still work). When you unfold them you have in effect a small tablet, each offering different ideas about how to do multi-tasking on the single unfolded screen.

There's a good case for doing things this way. Android software has evolved to make the most of the phone-slab form factor (for the moment at least iOS isn't in the folding frame). Some apps (I’m thinking of banking particularly) have lent themselves very well to the small screen size, and for that you still have a conventional phone screen.

But others not so much. The reality is that while phones have become the default choice for internet access for most people, compared to the larger screens of mainstream computers they offer a seriously impoverished user experience. It's not surprising then that designers should now be trying to move things on by adding the option of a fold out tablet screen, without compromising too much on the compact portability of a phone.

I was curious, so I recently bought a first generation Duo: because they're no longer supported you can get them fairly cheaply on eBay these days. I've learnt a few things.

The first is that Microsoft had a point. The Duo has (good) phone functionality in terms of being able to make and receive voice calls, but that's a secondary function. Many years ago Microsoft showed a concept called the Courier, a small notebook computer with two screens that would fold against each other. It would have had bespoke software, and never got launched, but many of us were intrigued by the prospect.

The Duo series is probably the closest we're going to get. It's best to think of it as a compact portable computing device, with two screens. You can span apps across both screens, when that's useful, but its primary mode is to have one app open on one screen and something else (whether another app or an adapted rendering of your home screen) on the other. This approach transforms portable productivity, and does it in a way that doesn't distort the way most Andoid apps are designed.

The hinge mechanism on the Duo is a thing of beauty, allowing both screens to fold back on themselves, so if you want to use it like a conventional phone, whether to make a call or type into a messaging app using one hand, you can do so quite comfortably. The screens are thin enough, the device light enough for this simply to work. When folded closed those screens are naturally protected.

For tablet-like use the gap between the two screens is less of an issue than you might think. I wouldn't generally choose to watch a film or sports games even on the enlarged single screen of a folding phone, but there are times when it's convenient and with the Duo the brain is good at filling in the information "lost" in the gap. It can be more of an issue when you try to read text across the gap, which is why in apps like Word the software will urge you to put the opened screens into a portrait position, so any lines of text won't be broken by the gap. It works.

Microsoft optimised many of its own apps for the dual screen configuration. For instance Outlook will divide itself across the gap with your inbox on one side and message preview (or perhaps your calendar) on the other. It's hit and miss with other spanned apps, though Google Maps works well.

The killer app, and the primary reason I wanted to buy a Duo, is for ebooks (and particularly the Kindle app). I love traditional paper books, but with a folding phone having two pages to view, across a decent screen width for each page, you have the closest I’ve seen to a paper experience, and now I wouldn’t try to read an ebook in any other way.

Which points to another key element. The aspect ratio on the Duo's screens is a little wider than on a normal slab phone. It makes for a far better reading device. the Pixel Fold has gone for a similar shape, and I only hope they have the courage to stick with it.

I bought the first generation Duo well aware of its shortcomings and with no intention of using it to replace my phone: I imagined I’d use it as a mini-tablet, without a SIM. But I found myself wanting to transfer my SIM almost immediately, to enjoy all of the different ways it could work (my Pixel watch covers the absence of NFC, 5G remains a practical irrelevance in most areas, and I'm not much of a fan of phone photography, though I'll admit this is a convenience I do miss).

So now I have a future dilemma. The Duo 2 remains a worthwhile and tempting upgrade, and I'd find it very hard to go back to a conventional single screen phone. But the market has moved in a different direction,  and the current crop of foldable phones don't have the same appeal. I have to hope that designers will continue to think about the strengths of the Duo series, and look for ways they could be incorporated in future devices. For the moment and for all its flaws I'll continue enjoying the most interesting and usable mobile device I’ve ever owned.