Allen and Greenough's New Latin Grammar, section 26

Heavy Construction

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26.

The Stem may be the same as the root: as in duc-is, of a leader, fer-t, he bears; but it is more frequently formed from the root -

1. By changing or lengthening its vowel: as in scob-s, sawdust (SCAB, shave); rég-is, of a king (REG, direct); vóc-is, of a voice (VOC, call).

2. By the addition of a simple suffix (originally another root): as in fugá-, stem of fuga, flight (FUG +á-); regi-s, you rule (REG +stem-ending eo-); sini-t, he allows (SI +neo-)[1][These suffixes are Indo-European stem-endings.]

3. By two or more of these methods: as in dúci-t, he leads (DUC + stem-ending eo-).

4. By derivation and composition, following the laws of development peculiar to the language. (See §§227 ff.)