Thorough is an adjective meaning complete and detailed: "They carried out a thorough inspection." Through is a preposition or adverb meaning from one side to the other, or finished: "The train goes through the tunnel." Thorough has two syllables and an extra <em>o</em>; through has one syllable and rhymes with <em>blue</em>.
The core difference
These two are separated by one letter and about as far apart in job as words get. Thorough describes how carefully something was done. Through describes movement or completion.
Through
Through is mostly a preposition — it needs something to go through:
- We drove through the village without stopping.
- She read through the contract twice.
- The news spread through the office by lunchtime.
It also works as an adverb ("let him through") and, informally, as an adjective meaning finished ("I'm through with excuses"). In American English it can mean "up to and including": Monday through Friday. British writers generally use Monday to Friday, which is worth knowing if you write dates for a mixed audience — through in that sense is unambiguous to Americans and slightly odd to British readers.
Thorough
Thorough is an adjective, and it sits before a noun or after a linking verb:
- The audit was more thorough than anyone expected.
- He gave the kitchen a thorough clean before the inspection.
- She's thorough — she'll find the error.
Its adverb is thoroughly ("thoroughly checked", "thoroughly enjoyed"), and it is the root of throughout's look-alike cousins thoroughfare and thoroughbred, both of which keep the two-syllable spelling.
The test
Say the sentence aloud. Through is one syllable — it sounds like threw. Thorough is two — THUH-ruh. If your ear hears two beats, the word needs the extra o.
If you would rather have a substitution test: swap in detailed. If the sentence survives, write thorough. Swap in across or from end to end; if that survives, write through.
The lookalikes and the real trap
The -ough family is the worst spelling cluster in English, and three neighbours cause most of the damage:
- though — meaning although: "It's late, though."
- trough — a long container, or a low point on a graph.
- tough — hard or difficult.
None of these can stand in for through or thorough. The mistake that slips past spellcheckers most often is "a through investigation" — every word is spelled correctly, so nothing flags it, and it means something absurd. Proofread for this one specifically; if you write thorough often, search your document for "through" before you send it.
Finally, throughout is built on through, not thorough: "reforms rolled out throughout the region."
Frequently Asked Questions
Is it "a through review" or "a thorough review"?
A thorough review — meaning a careful, complete one. "A through review" is a common typo that spellcheckers never catch, because through is a real word.
Is "Monday through Friday" correct?
It is standard American English and means Monday to Friday inclusive. British writers usually prefer "Monday to Friday", but the American form is clear and widely understood.
How do you pronounce thorough?
Two syllables: THUH-ruh in British English, closer to THUR-oh in American English. Through is one syllable and rhymes with blue.