THE COMMA Use a comma to separate 'non-defining relative clauses' from the main clause English has two types of relative clauses: non-defining (also called: commenting, non-essential) and defining (also called: essential, restrictive) relative clauses. This distinction is important if you are to punctuate these correctly in English. Unlike Finnish, a comma is only needed when the relative clause contains non-defining information. Notice the different comma usage in English and Finnish in the examples below. DEFINING INFORMATION 'Defining' relative clauses provide information that defines and restricts the meaning of the word that comes before the clause, and is thus 'essential' for understanding what we are talking about. In contrast to Finnish, defining relative clauses are never punctuated with a comma. Students Ø who have completed the course will be awarded a certificate. Opiskelijat, jotka ovat suorittaneet kurssin loppuun, myönnetään todistus. (Kurssin suorittaneille opiskelijoille myönnetään todistus.) (The absence of a comma in the English version signals that only those students finishing the course will be awarded a certificate, not all students!)   In formal definitions, the relative pronouns 'which', 'that' and 'who' are never introduced by a comma, because they are always defining to the meaning and define the class: A catalytic converter is a device in the vehicle exhaust system which/that chemically changes pollutants, such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into harmless substances. A scientist is a person who is expert in an area of science and uses scientific methods in research. A limited partnership is a business structure in which the liability of one or more of the partners is limited to the amount invested by him.   NON-DEFINING INFORMATION 'Non-defining' relative clauses comment or give extra details on the topic, and can thus be omitted from the sentence without losing the main meaning of the sentence. In fact, this is usually the best test of whether you are dealing with a non-defining or defining relative clauses. Non-defining relative clauses must also be 'separated' from the rest of the sentence by punctuation--most commonly by commas. In the examples below, notice how these versions differ in meaning from the 'defining' versions above. 'DEFINING' MEANING: My brother who lives in Germany is an Engineer. (I have two other brothers. The others live in the USA) Mexican food that/which is heavily spiced with chili gives me indigestion. (Not all Mexican foods are heavily spiced with chili. Only Mexican food containing chili give me stomach problems.) 'NON-DEFINING' MEANING: My brother, who lives in Germany, is an Engineer. (I have only one brother. He just happens to live in Germany) Mexican food, that which is heavily spiced with chili, gives me indigestion. (All Mexican food is heavily spiced with chili)   To make clear whether the information is necessary to describe the noun in an defining clause, use 'that' or 'which' with NO comma. To add extra information in a non-defining clause, use 'which' and be careful to add a comma.   A reminder: Don't put a comma in front of 'that', unless the word or phrase that needs to be separated by commas in front of 'that' (e.g., however). Compare the two examples below The evidence shows , however, that this variable does have a significant influence. The evidence shows Ø that this variable does have a significant influence.   'WHICH' OR 'THAT'? While 'that' can only be used to introduce defining information, 'which' is used to introduce both defining and non-defining information. Some grammarians still, however, insist on limiting 'which' to only non-defining relative clauses. Click the below for more details about the three special cases when 'that' cannot be used and 'which' is the only possibility.

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