One and the same vs. One in the same

One and the same

/wʌn ənd ðə seɪm/

Used to emphasize that two things are exactly the same or identical

Examples:

The two perspectives are one and the same in essence.Her pen name and real name were one and the same.The rules for all teams were one and the same.

Synonyms:

identicalsameequivalentselfsame

Antonyms:

differentdistinct

One in the same

/ˌwʌn ɪn ðə ˈseɪm/

A phrase used to emphasize that two things or persons are identical or very similar.

Examples:

People often think love and money are one in the same.Improving oneself and growing wiser are one in the same.She realized that her goals and dreams were one in the same.

Synonyms:

sameidenticalequivalent

Antonyms:

differentdistinctunlike

Ways to tell them apart:

• "One and the same" means that two things are exactly identical or the same.
• The phrase "one in the same" is often mistakenly used in place of "one and the same" but is incorrect.
• Remember that "one and the same" is a way to emphasize that there's no difference between two entities.
• There is no separate, established phrase as "one in the same" in proper usage.
• When you want to express identity or sameness, "one and the same" is the correct expression.