Beside means physically next to something: "She parked beside the church." Besides means in addition to, or apart from: "Besides the rent, I pay for parking." The extra s carries the extra meaning — addition, not position.
The core difference
Beside is about place. It puts one thing at the side of another, and it is nearly always interchangeable with next to: "He sat beside his mother." Besides is about addition or exception. It means as well as or, in a negative sentence, apart from: "Besides the deposit, there's a cleaning fee."
Besides has a second job that beside cannot do: it works as a linking adverb meaning anyway or in any case. "I don't want to drive. Besides, the car's low on fuel."
How to use each
- Beside — "The dog slept beside the radiator." "Her name appears beside mine on the certificate."
- Besides (preposition) — "Nobody besides Priya has read the report." "She speaks three languages besides English."
- Besides (adverb) — "It's too expensive. Besides, we already have one."
The test to run on your sentence
Try substituting next to. If the sentence still works, you want beside. If instead you can substitute in addition to, apart from or anyway, you want besides. "Sit next to me" works, so it's beside me. "In addition to the cost, it's slow" works, so it's besides the cost.
If you prefer a shorter memory hook: the extra s in besides is something extra added — and besides means in addition.
The exception worth knowing
One fixed phrase breaks the pattern: beside the point, meaning irrelevant. It takes beside, not besides, even though nothing is physically next to anything. "Whether he meant it is beside the point."
Two more idioms keep beside for figurative positions: beside himself (overcome with emotion — "She was beside herself with worry") and beside oneself with rage. Writing "besides himself" here is a genuine error.
The mistake people actually make
The common slip runs one way: writers use besides where they mean position — "He stood besides the door." That is wrong; it needs beside. The reverse error, using beside for addition, is rarer but does appear in formal writing: "Beside the fee, there is a levy" should be besides.
One usage note on register: besides as a sentence adverb is slightly informal. In a formal report, moreover, in addition or furthermore may read better. As a preposition, though, besides is perfectly formal.
Frequently Asked Questions
Is it "beside the point" or "besides the point"?
It is "beside the point". This is a fixed idiom meaning irrelevant, and it takes beside even though no physical position is involved. "Besides the point" is a common misspelling of it.
Can I say "besides that"?
Yes. "Besides that, everything went well" is correct and idiomatic, meaning apart from that. "Beside that" would mean physically next to that object.
Which one means "except"?
Besides, in negative sentences. "No one besides Anna knew" means no one except Anna. Beside never carries this meaning.