The Answerable Letter
A note to the future is read by someone who was not in the room: a grown child, a successor, you in five years, whoever finds the box. They will have the paper and nothing else. This page finds the words in your letter that only make sense today ("this", "next year", "you know the one"), asks what would settle any question the letter puts to them, and checks that it has two ways to arrive.
This opens on a made-up example: a friendly bet written on a birthday, to be opened two years later. Change it, or press "Start empty". Everything stays in this browser.
Words the reader cannot resolve
Each of these points at something that is obvious to you now and may be invisible later. Write what it means, or tick that it needs no saying (the letter already explains it).
If it asks something, what would settle it?
A bet, a promise, a "tell me if...", a decision to check on. The reader should not have to guess what counts as an answer.
Two ways to arrive
Addresses change, accounts close, phones get recycled, boxes get moved. For each route, say what would quietly break it. Two routes that break the same way are one route.
The letter, ready to be read cold
Why this exists
A real one, from the farm that made this page: a man made a bet on his birthday, out loud, that two years later "this might be a joke we laugh about over a beer." The bet was written down carefully, with a dated reminder and a backup path. What "this" meant, the joke two lines earlier, was not. The letter that will arrive asks the reader to settle a question about a joke it never quotes. Everything survived except the thing the sentence pointed at.
That is the common way letters to the future fail. Not lost; unreadable in one word.
This page runs entirely in your browser. It sends nothing anywhere and checks nothing on its own. It finds likely trouble words by pattern, so it will miss some and flag a few that are fine. Never put a password or account number in a letter; say where it is kept and who can open it.