Traditional grammar distinguishes neatly between two main chapters: on the one hand is the study of those combinations of significant elements that may involve some formal variations or accidents ('cow', 'cows', but 'ox', 'oxen', 'child', 'children'; 'work', 'worked', but 'keep', 'kept', 'sing', 'sang'), which normally take place within the word […] this is called accidence or morphology […].
- Martinet (1962), Pag. 89-90