Rule 3: Abbreviated units of measure

The singular and plural forms are the same. Do not add s or 's.

Examples

5 mg

8 mL

6 mcg

Rule 4: Irregular forms

Some words change form in the plural.

Rule 5: Irregular forms

Some forms are always singular in usage.

Examples

ascites

herpes

lues

Rule 6: Irregular forms

Some forms are always plural in usage.

Examples

adnexa

genitalia

menses