DOCBOOK: Language codes with countries

[Elliotte Rusty Harold <elharo@metalab.unc.edu>]

p. 120 of the Docbook book gives en_US as an example of an ISO 3166 country code for the lang attribute. However, these are normally written using hyphens rather than underscores; e.g. en-US. Which is correct for Docbook?

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| Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
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