1. Definition of Compound Nouns

A compound noun is formed by combining two or more words to create a new meaning. Examples include “mother-in-law,” “toothbrush,” and “classroom.”

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2. General Rules for Forming Plurals

• Pluralize the Main Noun: In most compound nouns, the main noun is pluralized.
  Examples:
• Singular: brother-in-law → Plural: brothers-in-law
• Singular: runner-up → Plural: runners-up
• No Main Noun: When there is no clear main noun, the entire word is pluralized by adding s to the end.
  Examples:
• Singular: toothbrush → Plural: toothbrushes
• Singular: check-in → Plural: check-ins
• Pluralize the Last Word: When the last word is the most significant, an s is added at the end.
  Examples:
• Singular: classroom → Plural: classrooms
• Singular: football → Plural: footballs

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3. Types of Compound Nouns

• Open Compound Nouns: These are written as separate words. The main noun is pluralized.
  Examples:
• Singular: editor in chief → Plural: editors in chief
• Singular: notary public → Plural: notaries public
• Hyphenated Compound Nouns: These are written with hyphens between words, and the main noun is pluralized.
  Examples:
• Singular: mother-in-law → Plural: mothers-in-law
• Singular: commander-in-chief → Plural: commanders-in-chief
• Closed Compound Nouns: These are written as one word, and s is added at the end to form the plural.
  Examples:
• Singular: classroom → Plural: classrooms
• Singular: football → Plural: footballs

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4. Exceptions

Some compound nouns change both parts when pluralized.
Examples:

• Singular: manservant → Plural: menservants
• Singular: woman doctor → Plural: women doctors

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5. Compound Nouns with Prepositions

For compound nouns that include prepositions, the main noun is pluralized.
Examples:

• Singular: passer-by → Plural: passers-by
• Singular: lookout → Plural: lookouts

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