note of hand vs IOU

note of hand and IOU both are nouns.

Word	Noun	Adjective	Verb	Adverb
note of hand	Yes	No	No	No
IOU	Yes	No	No	No

As nouns, IOU is a hyponym of note of hand; that is, IOU is a word with a more specific, narrower meaning than note of hand:

note of hand: a promise to pay a specified amount on demand or at a certain time
IOU: an informal debt instrument; representing `I owe you'

Other hyponyms of note of hand include demand note, note receivable, note payable, municipal note, time note.

note of hand (noun)	IOU (noun)
a promise to pay a specified amount on demand or at a certain time	an informal debt instrument; representing `I owe you'