Cause is a complete word meaning the thing that produces a result, or a movement people support — "the cause of the delay was a signal fault". Cos (and the apostrophe form 'cause) is an informal shortening of <em>because</em> — "we walked cos the bus never came". Cos is the usual British spelling; 'cause is more common in American writing.
The core difference
The trouble here is that one spelling is doing two jobs. Cause, written plainly, is a full noun and verb of its own. 'Cause is because with the first syllable clipped off, and the apostrophe marks the missing sound. British writers more often spell that same clipping cos (occasionally coz), which sidesteps the collision entirely.
Using cause
- "They never established the cause of the outage."
- "Damp is causing the paint to lift."
- "She gave a fortnight's pay to the cause."
- "There is no cause for alarm."
Using cos or 'cause
- "I'm not going, cos I've got work in the morning."
- "He said no 'cause he'd already booked something."
- "Just cos it's cheap doesn't mean it's good."
Both belong to speech, dialogue, lyrics, texts and deliberately casual writing. Neither is right for a report, an essay or a formal email — use because there.
The test you can run
Try replacing the word with because. If the sentence still works, you want cos or 'cause, never bare cause. If instead you can replace it with reason or make happen, you want plain cause with no apostrophe.
Memory trick
The apostrophe stands where the letters went. Because minus be leaves 'cause — the mark is a receipt for the missing syllable. No missing syllable, no apostrophe.
Being honest about the apostrophe
Writing bare cause for because — "I left cause I was bored" — is extremely common in messaging and social media, and it is not going away. It is still treated as an error by editors, mainly because it garden-paths the reader for a beat: the eye takes cause as a noun before the sentence corrects it. Cos has no such problem, which is a fair argument for preferring it.
Some style guides also dislike the apostrophe form on the grounds that clipped words settle down without one over time (as phone did). That is a preference rather than a rule; both cos and 'cause are recorded in major dictionaries.
The other cos
Two unrelated words share the spelling. Cos is a variety of lettuce, better known in some places as romaine, and cos is the mathematical abbreviation for cosine. Context separates them easily enough, but it is worth knowing before you sprinkle the word through a recipe.
Frequently Asked Questions
Is "cause" without an apostrophe wrong when it means because?
In edited writing, yes — use because, 'cause or cos. In informal text it is very widespread and nobody will misunderstand you, but it does read as a slip rather than a style.
Cos or coz?
Cos is far more common in British writing and is the form dictionaries list first. Coz turns up in dialect writing and older texts, where it can also be an old clipping of cousin.
Can I use cos in an essay?
No. Write because. Cos, 'cause and coz all mark casual speech, and an examiner or editor will treat them as register errors even though the meaning is clear.