421

Why 421 is special

Same number, different meanings

How 421 is understood across different contexts — never implying one meaning because of another.

• Mathematics

  421 is a consequence of connection reuse. HTTP/2 lets a browser send requests for several hostnames down one connection when their certificates match, and 421 is how a server says this particular hostname is not one it serves — a signal to open a new connection rather than an error the user caused.

• Technology & computing

  HTTP/2 lets a browser reuse one connection for several hostnames whose certificates match. 421 is the server saying this hostname is not one it can answer for, and the correct client response is to open a fresh connection rather than to show the user an error.

• Media & popular culture

  The Sennheiser MD 421 is a large-diaphragm dynamic microphone with a five-position bass roll-off, standard on tom-toms, guitar cabinets and broadcast speech since 1960.

Not just a mathematical number

421is also used in ways where ordinary arithmetic doesn't apply — each listed below with what genuinely does and doesn't follow from it.

• Mathematical number

  A genuine quantity — its mathematical properties (prime, square, factors, and so on) are real facts about it.

  A genuine quantity that ordinary arithmetic applies to — it can be prime, even, a perfect square, added to another number meaningfully, and so on.

• Code

  Also used as a code — HTTP status codes.

  An assigned identifier from a fixed registry or standard — its digits don't represent an amount, and arithmetic on it is meaningless (HTTP status 404 + 1 is not a 'better' status; it's a different, unrelated code).