So, this is pure curiosity. I've heard the term 3rd party, as in, "3rd party library" for quite a while.

I wonder why we say 3rd and not 2nd (or 4th)?

Who are the first and second parties and where did the term come from?

  • Third party is anyone "not party to the contract". Therefore, it depends on what "contract" you're referring to.
    – rwong
    Apr 24 '18 at 3:09
  • A "third party" in software usually means an upstream supplier of software, in relation to whom you (or your firm) are the client. This being distinguished from yourself as a supplier of software, and your clients in turn.
    – Steve
    Oct 31 '19 at 8:13

You, the developer, are the first party.

The customer/consumer of your application is the second party.

Anyone else that supplies libraries, icons, or other resources is a third party (in the sense of "third" meaning some other outside party).

See Also

  • Thanks. Is this a known convention? Was it borrowed from another discipline? Jul 29 '14 at 23:09
  • 3
    This is not correct -- the second party is the primary vendor of the system you are using; the "third party" is anyone else making compatible products for that system; admittedly, this is less important a distinction in this age of open systems!
    – jimwise
    Jul 30 '14 at 2:49

I think it's from contract language.

You, the 1st party, may create a contract with someone, the 2nd party, to undertake some piece of work. They may be an expert in one area of the project, but not in another and so they may choose to sub-contract that to a 3rd party.

Edit to add - In contract law you need two parties to create a contract, you will generally think of yourself as the first party and the other party to the contract as the second, but that's just a matter of perspective. One is not labelled as such. Anyone not directly involved in the contract is then a third party.

  • It does make sense in a subcontracting model. Didn't think of that. Jul 29 '14 at 23:11

First party is the user of the software, second party is the producer or developer, third party is another producer/developer who adds to the second party's work already.

The current answer chosen is incorrect as @jimwise already indicated.

  1. First Party - User
  2. Second Party - Developer/Software Producer
  3. Third Party - Additional Developer/Software Producer - E.g Widgets for web applications.

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