That allowed me to create this visualization -- to create software that allowed me to see how this was all moving in a city. Then we started adding in the next element, which are taxi cabs. Now we have another entity roaming about the metropolis, reporting where it is and what work it has, going over GPS and CB radio or cellphone. And then you get to the emergency services: ambulances, firetrucks and police -- and suddenly you have have this very rich sense of what’s happening right now in the city.

Twitter has been my life's work in many senses. It started with a fascination with cities and how they work, and what’s going on in them right now. That led me to the only thing that was tractable in discovering that, which was bicycle messengers and truck couriers roaming about, delivering packages.

It has very special significance -- it's hanging in the office somewhere with one other page. Whenever I'm thinking about something, I really like to take out the yellow notepad and get it down.

It was when Dorsey saw these systems through the eyes of the social, mobile Web, where anyone can squawk from anywhere, that Twitter's Big Idea was born.

Twitter didn't just fly out of thin air and land on a branch . As Dorsey explains, it has conceptual roots in the world of vehicle dispatch -- where cars and bikes zooming around town must constantly squawk to each other about where they are and what they're up to.

The legal-pad sketch of the idea that would become Twitter has been noticed before , but given all the recent hype, we thought we'd track down Dorsey and ask him about it in a little more detail. In the following interview, Dorsey uses the document to touch on aspects of the micromessaging service's history, including the inspirations and constraints that came to define one of the Web's most rapidly growing information channels.

Sitting in the Flickr archives is a nearly 10-year-old document uploaded a couple of years ago by its author, Jack Dorsey ( @jack ), who started Twitter in 2006 along with co-founders Evan Williams ( @ev ) and Biz Stone ( @biz ).

... was instant messenger. This aspect where you can just locate your buddy list and at a glance locate what your friends are up to, or what they say they’re up to. I found the same parallels in dispatch -- here’s a bunch of ambulances and couriers reporting where they are, and here’s my friends. Now, the problem with IM is that you’re bound to the computer, so it really limited the interestingness of the messages.

So that document was around 2000-2001 when I really got into IM and a service called LiveJournal. And it was crystallizing the thought: What if you have LiveJournal, but you just make it more live? You have these people watching your journal, but it all happens in real time, and you can update it from anywhere. That document was an exploration of that concept.

When did you first try to build out the idea?

I tried it back in 2000 with the first device that RIM made -- the RIM 850, which was the predecessor to the BlackBerry. A very simple squat little e-mail device. It had four lines of text and a typical BlackBerry keyboard. They were like $400, and it would just do e-mail. I wrote a very simple program to listen to an e-mail address and take any updates from me and send them out to a list of my friends. And my friends could reply to that e-mail and tell me what they’re doing.

But the problem was that no one else had those devices –- so again, it limited the experience of that. We were limited until 2005-2006 when SMS took off in this country and I could finally send a message from Cingular to Verizon. And that just crystallized -- well, now’s the time for this idea. And we started working on it.

It was really SMS that inspired the further direction -- the particular constraint of 140 characters was kind of borrowed. You have a natural constraint with the couriers when you update your location or with IM when you update your status. But SMS allowed this other constraint, where most basic phones are limited to 160 characters before they split the messages. So in order to minimize the hassle and thinking around receiving a message, we wanted to make sure that we were not splitting any messages. So we took 20 characters for the user name, and left 140 for the content. That’s where it all came from.

For any potential Twitter historians out there, can you offer a few more details about the drawing -- the little googly eyes, for example?

The little eyeballs were "watching." The concept was watching before we kind of switched it and developed it into "following." So you could watch or unwatch someone -- but we found a better word -- follow or unfollow. The important consideration there was that on Twitter, you’re not watching the person, you’re watching what they produce. It’s not a social network, so there’s no real social pressure inherent in having to call them a "friend" or having to call them a relative, because you’re not dealing with them personally, you’re dealing with what they’ve put out there.

The document's user interface metaphor is very similar [to how Twitter turned out]. You have a little box to "set" your update, and past updates would go down into the timeline below.

Immediately the idea was device-agnostic. You could deliver over e-mail or deliver over Jabber, because IM was a real-time mechanism -- and eventually you could deliver over SMS as well. And the only other aspect on that page was how to find other people. If you know someone, you type in their name or e-mail address, and you can immediately start following their updates.

What are the "authentication triples" on the upper left there?

I was trying to be a little bit too smart, and was trying to figure out ways to do everything without a password. But that’s very difficult and requires way too much thought. So we punted on that. But someone will figure it out. [laughs]

Then when did the service's name morph from “Status/Stat.us” to “twittr” to Twitter?

The working name was just "Status" for a while. It actually didn’t have a name. We were trying to name it, and mobile was a big aspect of the product early on ... We liked the SMS aspect, and how you could update from anywhere and receive from anywhere.

We wanted to capture that in the name -- we wanted to capture that feeling: the physical sensation that you’re buzzing your friend’s pocket. It’s like buzzing all over the world. So we did a bunch of name-storming, and we came up with the word "twitch," because the phone kind of vibrates when it moves. But "twitch" is not a good product name because it doesn’t bring up the right imagery. So we looked in the dictionary for words around it, and we came across the word "twitter," and it was just perfect. The definition was "a short burst of inconsequential information," and "chirps from birds." And that’s exactly what the product was.

The whole bird thing: bird chirps sound meaningless to us, but meaning is applied by other birds. The same is true of Twitter: a lot of messages can be seen as completely useless and meaningless, but it’s entirely dependent on the recipient. So we just fell in love with the word. It was like, "Oh, this is it." We can use it as a verb, as a noun, it fits with so many other words. If you get too many messages you’re "twitterpated" -- the name was just perfect.

But you needed that short code -– in order to operate SMS you need the short code to operate with this cellular administration. So we were trying to get "twttr" -- because we could just take out the vowels and get the 5-digit code. But unfortunately Teen People had that code -– it was ‘txttp’ [Text TP]. So we just decided to get an easy-to-remember short code [40404], and put the vowels back in.

So Twitter was it, and it’s been a big part of our success. Naming something and getting the branding right is really important.

Updated Feb. 19: Here's the second part of the interview, in which Dorsey talks about the growing Twitter ecosystem, the service's effect on news gathering and why he doesn't like to "go back in time."

-- David Sarno [follow on Twitter]