I’ve only defined it on tagdef.com though, any others out there continue to lack a definition.

Defining the tag that way also doesn’t prevent others from using it for another purpose, or from abusing it with spam. It confers no degree of uniqueness or ownership. And, where more than one definition exists, it provides no mechanism for deciding which bits of content were referring to which definition when they were tagged.

Centralised approaches, and approaches that need prior knowledge of a specific service, are less useful on the web than mechanisms that are de-centralised and discoverable through the normal mechanism of following a link…

We already have a great, well-understood, scalable, de-centralised naming and lookup mechanism. It’s called the web. Why tey to invent another on top of it – which is what hashtags are.

I agree with you, and with Andy Powell’s comments on twitter that at this point putting URLs into twitter, or using them as tags on Flickr, is harder than using simple word tagging. But, the mechanisms for adding links to stuff are all over the place in other tools, there are well understood interaction design patterns to make that easy.

With good UI design we could be using the web for tagging, rather than inventing a different linking mechanism on top of it.

For now, I’ve decided that I’m blogging!

Ambiguity in the moment can be addressed by the adoption of dictionaries/registries; you can then synthesize a GUID with a timestamp if you need it- a similar approach can be used for airline flights, which I think is a much better analogy than what I thought of. For both airline flights and twitter hashtags, the overriding consideration has to be user acceptance.

In your list of requirements, 5 (”That the tag can be used anywhere on the web easily”) has to come first – and it has to come first by quite a long way… because otherwise you just have something which isn’t usable and therefore won’t get used.

Still, I suppose that one way of ensuring uniqueness is to make sure that no-one ever bothers to create tags in the first place!? :-)

I’m not really suggesting using a broader character set and GUIDs as that has some real issues. It was really an exploration of what we really want from tagging, which in my mind comes down to tagging with URIs as Linked Data.

I do think ambiguity over time is sometimes an issue, but ambiguity in the moment is also an issue and comes from the tension between uniqueness and length that twitter introduces.

I don’t think that’s the same as saying longitude is ambiguous over latitude, it’s more like saying my surname alone is not enough to identify me. Or perhaps, a flight number is not enough to catch the plane, you also need to know the date and time…

My reference to ASCII was simply that the usual way to represent a GUID is as an ASCII encoded hexadecimal string, but that’s not the only way you can do it. I’m fully aware that UTF8 is an encoding, I’m not worried about the language being sloppy as it’s not important for the point of this post.

You’re quite right that Unicode uses a 2 byte character id rather than a 4 byte one, my bad as I was working with the hex characters visually, I’ve corrected that now.

Fundamentally, though, my point is that hashtags are a way of marking content that could be done better using URIs. If people started using URIs you can be sure that UIs would evolve to make it easier too.

Let me get this straight: you think hashtags are broken because they are ambiguous over time? Isn’t that a bit like saying longitudes are broken because they’re ambiguous over latitude? I compile some data about conference hashtags: http://go-to-hellman.blogspot.com/2009/06/conference-hashtags-dont-evolve.html

Also, remember that utf8 is an encoding, not a character set, so there’s no such thing as a UTF8 character. Unicode characters are 2 bytes, not 4, and please don’t mix up ascii and hexadecimal.

Isn’t blogging wonderful, you can use your readers as fact-checkers?

In terms of the devices, I wonder how long that issue will persist given how mobile devices are becoming more and more capable web tablets.