These are often binary or unary conditions which act as constraints in various forms of linguistic analysis.
In phonology, segments are categorized into natural classes on the basis of their distinctive features.
Each feature is a quality or characteristic of the natural class, such as voice or manner.
English pronouns also have the number feature, which can have a value of either "singular" or "plural".
[1] In formal models of grammar, features can be represented as attribute-value pairs.