[1] [CITE@en[GNU `gettext' utilities]] ([TIME[2009-03-20 11:59:11 +09:00]] 版) <http://www.gnu.org/software/gettext/manual/gettext.html#PO-Files>

[2]
>The letters PO in .po files means Portable Object, to distinguish it from .mo files, where MO stands for Machine Object. This paradigm, as well as the PO file format, is inspired by the NLS standard developed by Uniforum, and first implemented by Sun in their Solaris system. 

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[CITE@en[GNU `gettext' utilities]] ([TIME[2009-03-20 11:59:11 +09:00]] 版) <http://www.gnu.org/software/gettext/manual/gettext.html#index-files_002c-_0040file_007b_002epo_007d-and-_0040file_007b_002emo_007d-29>