405

Why 405 is special

Same number, different meanings

How 405 is understood across different contexts — never implying one meaning because of another.

• Mathematics

  405 distinguishes the address from the action. A URL that answers a GET may refuse a DELETE, and 405 says exactly that rather than pretending the resource is missing. The specification requires the response to list which methods are allowed, which makes it one of the few error codes obliged to be helpful.

• Science

  IC 405 is illuminated by AE Aurigae, a runaway star ejected from the Orion region that is passing through the nebula rather than having formed in it.

• Technology & computing

  The specification requires a 405 response to include an Allow header listing the methods the resource does accept. That makes it unusual: most error codes describe a failure, while 405 is obliged to describe the alternative. An API returning a bare 405 is technically non-compliant.

Not just a mathematical number

405is 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).