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Written legal documents; created to effect the rights of the parties, given formal expression to a legal act or agreement for the purpose of creating, modifying or terminating a right. ...

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in legal translation the term مستند is for document and instrument is for وثيقة.

The main point here is that "instrument" in this context is no more than a fancy term for "document" - nothing more. A document could be a contract, a will, a check, or the like. In other words, it is not a document in the sense that we use when we refer to some computer files (like a Word document). It is rather a document that has a legal consequence and whose force is effected by a signature or signatures.

Now, I have seen some translators render this term

آليات، أدوات

I am not going to comment on such translations, and I am not going to favor any of the three terms I suggested. My point here is simply to point out the almost obvious: an instrument is a document, and that is all.

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Note added at 1 hr (2008-08-31 08:16:26 GMT)
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Please check out this page:

http://en.wikipedia.org/wiki/Legal_instrument

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