Here are still other ways of forming plural nouns.

1. Form the plural of special nouns by changing their spelling.

Example:

• foot - feet

2. Some nouns have the same singular and plural forms.

Example:

• sheep - sheep

3. Form the plural of compound nouns by adding -s to the more important or most important word.

Examples:

• passerby - passersby
• maid-in-waiting - maids-in-waiting
• follow-up - follow-ups
• commander-in-chief - commanders-in-chief

4. Form the plural of capital letters and numerals by adding -s.

Examples:

• 8 - 8s
• 1970 - 1970s
• K - Ks

5. Form the plural of small or lowercase letters, abbreviations/initials with periods, and abbreviations/initials with both capital and lowercase letters by adding an apostrophe and an -s. (-'s)

Examples:

• k - k's
• PhD - PhD's