Learn English Through Pictures

Arm

Each of the two upper limbs of the human body from the shoulder to the hand.

Chair

A separate seat for one person, typically with a back and four legs.

Armchair

A comfortable chair, typically upholstered, with side supports for the arms.

Example Sentence:-

"She sat in a deep armchair reading a book ."

!Note - We sit on a chair, unless it's an armchair, and then we sit in it.