1911 Encyclopedia Britannica
I.O.U. ("I owe you"), a written acknowledgment of a debt. It usually runs thus: To. I.O.U. pounds.
(Signed) Date An I.O.U., if worded as above, or even if the words "for value received" are added, does not acquire a stamp, as it contains no terms of agreement. If any such words as "to be paid on such a day" are added, it requires a stamp. An I.O.U. should be addressed to the creditor by name, though its validity is not impaired by such omission. Being a distinct admission of a sum due, it is prima facie evidence of an account stated, but where it is the only item of evidence of account it may be rebutted by showing there was no debt and no demand which could be enforced by virtue of it. An I.O.U. is not negotiable.
These files are public domain.
Chisholm, Hugh, General Editor. Entry for 'I.O.U'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/i/iou.html. 1910.