A capitonym is a word that changes its meaning (and sometimes pronunciation) when it is capitalized, and usually applies to capitalization due to proper nouns or eponyms.^{[citation needed]} It is a portmanteau of the word capital with the suffix -onym. A capitonym is a form of homograph and - when the two forms are pronounced differently - also of heteronym. In situations where both words should be capitalized (such as the beginning of a sentence), there will be nothing to distinguish between them.

Although some pairs, such as march and March, are completely unrelated, in other cases, such as august and catholic, the capitalized form is a name which is etymologically related to the uncapitalized form. For example, August derives from the name of Imperator Augustus, who named himself after the word augustus, whence English august. And both Catholic and catholic derive from a Greek adjective meaning "universal". Pairs in which one word is simply a secondary meaning of the other - e.g. Masonry (secret society), which is essentially a peculiar use of the word masonry (wall building) - are omitted.

In other languages there are more, or less, of these pairs depending on that language's capitalization rules. For example, in German, where all nouns are capitalized, there are many pairs, such as Laut (sound) ~ laut (loud) or Morgen (morning) ~ morgen (tomorrow). In contrast, in Italian, as well as Spanish, almost no words (except proper names) are capitalized, not even months, weekdays nor ethnical adjectives, so there are extremely few, if any, such pairs. (An example is Messa (Mass) ~ messa (feminine past participe of mettere = to put), though the former is sometimes spelled with a lowercase m too.)

Some question the correctness of certain case-sensitive distinctions that others make. Examples are church (a building) and Church (an organization), mass (physical quantity) and Mass (liturgical function). In the Soviet Union, it was forbidden to write God with an upper-case letter, making impossible this manner of distinguishing God (the being believed to create and rule the universe) from god (one of beings believed to control some part of the universe).[1]

Examples in poems

The following poems, of unknown origin, are examples of the use of capitonyms:

Job's Job
In August, an august patriarch
Was reading an ad in Reading, Mass.
Long-suffering Job secured a job
To polish piles of Polish brass.

Herb's Herbs
A herb store owner, name of Herb,
Moved to a rainier Mount Rainier.
It would have been so nice in Nice,
And even tangier in Tangier.