Latin words in -gō, -ginis seen in -īgō, -ūgō, and -āgō (e.g., rōbīgō ‘plant rust’, aerūgō ‘copper rust’, and imāgō ‘likeness, image’ respectively) have often been compared to words with the substantivizing suffix -k- seen in Latin words ending in -ex, -ix, -ēx, -īx, -āx, and perhaps -ōx (e.g., senex ‘old man, old’, fornix ‘arch, vault’, vervēx ‘wether’, fēlīx ‘fruitful’, fugāx ‘prone to run away, fugitive’, and perhaps vēlōx ‘swift’ respectively). However, this thesis argues that the two endings are historically unrelated. This is supported by the lack of word pairs in -k- and -gō, -ginis as well as the different functions of these two morphemes. The Latin suffix -k- made nouns from adnominals, and -gō, -ginis seems to be the reflex of the second member of an inherited compound. It is more specifically argued, based on the semantics of the surviving -gō, -ginis forms: (1) that -gō, -ginis goes back to *-h$_1$g$^w$-o- ‘shine (forth), appear’ plus a familiar PIE “individualizing” *-(o)n- suffix (e.g., Latin *h$_1$roud$^h$o- ‘red’ [> Lat. rūfus] → h$_1$roud$^h$o-(o)n- [> Latin rūfōn- ‘redhead’]); (2) that the comparative evidence of Greek ἐρυσίβη ‘plant rust’ (ultimately reflecting a *h$_1$rud$^h$i-h$_1$g$^w$-o-) justifies *h$_1$ro/eud$^h$i-h$_1$g$^w$o-(o)n- for rōbīgō ‘id.’, which thus becomes a key indication of the origin of the whole -īgō type; (3) that there is a plausible path for the phonological development of *h$_1$ro/eud$^h$i-h$_1$g$^w$o-(o)n- >> -gō, -ginis.