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Plural of Thesis

The Quick Answer
The plural of thesis is theses.

The Plural of Thesis

The plural of thesis is theses.
  • PhD theses are usually over two hundred pages.
  • Copyright permission from the author is required before theses can be digitised.
The noun thesis has a Greek root, which is the derivation of the plural theses. There is no alternative English plural form. The noun thesis adheres to the standard rules for forming the plurals of nouns in English (shown in the table below).

Type Example of Type Forming the Plural Plural
Most Nouns lamp
scythe
add s lamps
scythes
Noun Ending s, sh, ch, x or z box
dress
add es boxes
dresses
Nouns ending [consonant] o zero
tomato
add either s or es
(There are no rules for this - you have to know.)
zeros
tomatoes
Nouns ending [vowel] o patio
ratio
add s patios
ratios
Nouns ending [consonant] y story
penny
change the y to an i and add es stories
pennies
Nouns ending [vowel] y donkey
chimney
add s donkey
chimneys
Nouns ending f or fe dwarf
hoof
ves and/or s
(There are no rules - you have to know.)
dwarfs
hooves or hoofs
Nouns ending is thesis
crisis
change the is to es theses
crises
Exceptions man
louse
some nouns undergo a vowel or letters change men
lice
More exceptions salmon
sheep
some nouns do not change at all salmon
sheep
Foreign rulings fungus
medium
some nouns adopt foreign rulings fungi
media

Why Is There Confusion over the Plural of Thesis?

Theses is the only way to make the noun thesis plural.

Confusion arises because some mistakenly believe that all nouns ending in s should form a plural that adds es to the end of the word. When a noun ends with is, you need to replace the is with an es to form the plural. This is because its plural form derives from Greek.