Morphological structures may be simple or complex. Simple structures consist of a single morpheme, free or bound. [...] Simple structures may be called 'monomorphemic' [...]
- Nida (1949), a pag.97

A simple structure always consists of a root. This may be potentially free, e.g. 'boy', 'John', 'run', and 'up', or it may be bound. A bound root may be either a stem, e.g. '-ceive', '-tain', '-veal' (cf. 'receive', 'retain', 'reveal') or a clitic, e.g. /ə-, -ə, -l/, atonic forms of 'a', 'you', and 'will'.
- Nida (1949), a pag.97