That or which?

that, which


That and which. These words get mixed up quite a bit, with many users either assuming they are synonyms or not understanding the difference. That introduces essential information while which introduces supplemental information. Think of that as part of a long noun cluster, all of which is required to understand the sentence, and which as a word that precedes information that's enlightening but not necessary.

For example, if I tell you that I live at the fourth house, which is single story, from the corner, you would count one, two, three, four houses, and -- yup, it's a single story one. If I say I live in the fourth house that is single story from the corner, you count the single story houses only.

Here it is a bit more visually:

I live in the (fourth house), which is single story, from the corner.

I live in the (fourth house that is single story) from the corner.

Which is also generally set off by comma, but that won't help you if you're writing and don't know the difference or if the writer you're reading didn't know the difference.