Sturtevant's Law: The tendency for Proto-Indo-European voiced (and voiced aspirated) stops to be represented in Hittite by single consonants (*yugom = i-ú-kán) and for PIE voiceless stops to be spelled with double consonants (*k'eyto > ki-it-ta).
This may or may not be a meaningful distinction in the orthography; from what I've seen, Hittite was rather haphazardly spelled.
This probably was not meant to indicate voicing, as the Hittite syllabary was borrowed from Akkadian, which did have signs for voiced and voiceless stops, which Hittite used interchangeably.
It may have indicated length (which would account for double writing of other consonants, such as nasals), but the existence of a common first person plural form in -um-me-ni seems to indicate otherwise, if -meni is supposed to be formed from the original PIE ending *-mos.
This law is apparently named after the Hittitologist Edgar Sturtevant.