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Twitter was based on sms, the standard SMS character limit is 160. They used 140 so they could use the remained 20 chars for other purposes.

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140 and 160 are related when it comes to SMS.

The GSM-7 alphabet is the most common one in use with SMS (or, at least, it was as UCS-2 is more common now with emojis and such).

160 is the number of GSM-7 characters.

160*7/8 = 140 which is the number of bytes in the userdata portion of the TPDU.

I don't think the Twitter choice of 140 was anything to do with this though and is just a coincidence. Back during dumbphones the only way to receive tweets while mobile was via the texting interface, and it would want to prepend the username. I don't think reserving 20 for the username has anything to do with how many bits are used to represent the alphabet.

That's coincidence, though. I used Twitter to keep in touch with friends via SMS in 02008, and the messages had space for a prelude to say who they were from. In the opposite direction, you could use that space to tell Twitter to send the message privately to someone.

The username length restriction might come partly from that. They could surely relax it by now, though. I saw it at play this week when @SecondGentleman (15 characters) changed to @SecondGent46.

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