Each of the following written promises is signed by its maker. Which of them is a promissory note under the Negotiable Instruments Act, 1881?

Section 4 requires an unconditional undertaking to pay a certain sum, and section 5 provides that a promise is not conditional merely because payment falls a fixed period after an event certain to happen, such as a death, though its date is uncertain. A promise tied to the maker's marriage depends on an event that may never happen, and a promise to pay on death only if enough is left is conditional, so neither is a promissory note. A promise that adds 'all other sums due' fails because the amount is not certain.