The "Pedersen system" (he immodestly said) is
<verb>
<verb>-g- actor
<verb>-d- object
eg. *k-r- "turn"
the *k-r-g- (eg "ring") *k-r- 's ("encloses") the *k-r-d- (Latin
cardo "axis", or cor, cordis "heart")
Note that in Latin third declension consonant stems (those of them
having a suffixed consonant outside of nom.sg., I don't believe
nom.sg. lost that consonant) include stems in -d-, but not in -g-.
Those in -g- have -g- also in nom.sg. (eg. rex, regs).