Fourth is the ordinal number: "she finished fourth" or "the fourth of July." Forth means forward, onward or out: "they set forth at dawn" and "back and forth." If the sentence is about position in a sequence — fourth in line, the fourth floor — you need the u. (<em>Forty</em> is the exception that catches everyone: it comes from four but drops the u.)
The core difference
Fourth is a number word, built straightforwardly on four. Forth is an adverb of movement, meaning outward or onward, and it survives mostly in fixed phrases. They sound identical in most accents, which is why the confusion is entirely a written one.
Using fourth
Fourth does everything an ordinal number does: it ranks, dates and divides.
- This is the fourth time I've asked.
- He lives on the fourth floor.
- Cut the cake into quarters and give each child a fourth.
- The fourth quarter results were disappointing.
Using forth
Forth rarely stands alone in modern English. You will nearly always meet it inside a set expression:
- back and forth — the emails went back and forth for a week
- and so forth — bread, milk, eggs and so forth
- set forth — the terms set forth in clause 6
- bring forth, put forth, sally forth, henceforth, thenceforth
Outside these, forth sounds deliberately archaic or elevated: "Go forth and multiply" belongs to scripture, not to a work email.
The test
Ask whether the word has anything to do with the number four. If it does, keep the u — four is hiding inside fourth, and it is the only spelling clue you need. If the meaning is forward, out or onward, drop the u.
A second check works well for the tricky pairs: if you could substitute forward or onward without breaking the sentence, write forth. "The debate went back and forward" is clumsy but comprehensible, which confirms forth is right there.
The mistakes to watch for
The single commonest error is writing "and so fourth" for and so forth. Nothing is being counted, so there is no u. Close behind is "back and fourth," which describes movement, not position in a queue.
Running the other way, "the forth of July" is a fairly frequent slip in American writing. The date is the fourth day of the month, so the u stays.
Two extras: the Firth of Forth is a Scottish estuary and a proper name, and forte is a different word again — meaning either a strength ("maths was never my forte") or, in music, loudly.
Frequently Asked Questions
Is it 'and so forth' or 'and so fourth'?
And so forth. The phrase means "and onward from there" — nothing is being numbered, so there is no u. The same applies to back and forth.
Which spelling for the American holiday?
The Fourth of July, with a u. It is the fourth day of the month, so the ordinal number is required. "Forth of July" is a misspelling, however often it appears on greetings cards.