Two morphemes which are structurally significant in a single structural series may be called 'supplementary morphemes'.
- Nida (1949), a pag.73

Supplementary morphemes have identical meaning and overlap in distribution with the same external distribution class. For example, in the past-tense formation there are the supplementary morphemes /e ← iy/ : 'bled', 'read', 'met', and /-d ~ -t/ : 'flowed', 'walked', with supplementation in /fled/ 'fled' and /kept/ 'kept'. Supplementary morphemes occurring in the same form comprise a single immediate constituent. That is to say, the immediate constituents of 'kept' are the nuclear /k...p/ plus the complex peripheral /e ← iy + -t/.
- Nida (1949), a pag.110