A language requires that overt plural markings are used to indicate any instance of noun pluralization.  In other languages, such markings may be always optional or optional for inanimate nouns only.

<br><br><b>Reference</b><br>
Haspelmath, Martin. 2008. Occurrence of Nominal Plurality.
In Haspelmath et al (2008), chapter 34.
Available online at <a href="http://wals.info/feature/34">
http://wals.info/feature/34</a href>.
