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### 3.6 `xr`: Cross-Document References

The LaTeX package `xr` makes it possible to create references to labels defined in external documents. The preamble of a document using `xr` will contain something like this:

         \usepackage{xr}
         \externaldocument[V1-]{volume1}
         \externaldocument[V3-]{volume3}

and we can make references to any labels defined in these external documents by using the prefixes ‘`V1-`’ and ‘`V3-`’, respectively.

RefTeX can be used to create such references as well. Start the referencing process normally, by pressing `C-c )`. Select a label type if necessary. When you see the label selection buffer, pressing `x` will switch to the label selection buffer of one of the external documents. You may then select a label as before and RefTeX will insert it along with the required prefix.

For this kind of inter-document cross-references, saving of parsing information and the use of multiple selection buffers can mean a large speed-up (see Optimizations).