“Using which instead of that. That introduces essential clauses while which introduces nonessential clauses. Consider the sentence “Tools that have sharp edges can cause nasty cuts.” If you remove the words “that have sharp edges,” the sentence loses much of its meaning. The clause is essential. Now consider “Roses, which come in many colors, have thorns on their stems.” You can remove “which come in many colors” and the meaning of the rest of the sentence is intact. The clause is not essential. Another way to remember: If the clause obviously needs to be set off by commas, use which.”