That vs. which
SAT glossary · Course Foundations
Definition
In edited American prose 'that' introduces essential clauses and takes no comma; 'which' introduces non-essential clauses and takes a comma. 'The samples that were contaminated were discarded' means only the contaminated ones; 'The samples, which were contaminated, were discarded' means all of them.
That vs. which in plain English
In properly edited American writing, 'that' introduces ESSENTIAL information (needed to know exactly which thing you're talking about) and never takes a comma before it; 'which' introduces NON-ESSENTIAL, extra information and always needs a comma before it. Compare: 'The samples THAT were contaminated were discarded' (meaning only SOME specific samples — the contaminated ones — got thrown out) versus 'The samples, WHICH were contaminated, were discarded' (meaning ALL the samples were contaminated, and that fact is just extra background information, and all of them got thrown out).
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