The plural suffix "-s" can also influence the form taken by the preceding morpheme, as in the case of the words "leaf" and "knife", which end with in the singular/but have in the plural ("leaves", "knives"). On a morphophonological level, the morphemes may be analyzed as ending in a morphophoneme , which becomes voiced when a voiced consonant (in this case the of the plural ending) is attached to it. The rule may be written symbolically as -> / [αvoice. This expression is called Alpha Notation in which α can be + (positive value) or − (negative value).