Lindeman's Law: an attempt to explain certain forms in Rigvedic Sanskrit—actually a restriction of Sievers' Law to originally monosyllabic forms.
Essentially: Proto-Indo-European *y and *iy interchange, as do *w and *uw, the longer forms occurring after heavy syllables, thus preventing breaking up a series like *VN CGV to a more manageable *VN CVGV.
In practical terms, this results in Sanskrit pairs like dyāus, diyāus "sky".
The conditioning factors of this rule imply that the long forms should be much more common than they actually are, and certainly more common than the short forms; however, this is only true of duva- "two".