The Comma ( , comma)

The comma has many uses and is one of the most important punctuation marks.

1. Comma for lists

   One of the simplest uses of a comma is to separate items in a list.

   • Patterns like these begin to form at a small scale when materials undergo processes like dryingfirst item in the list, comma separating items in the list freezingsecond item in the list, comma separating items in the list wrinklingthird item in the list, comma separating items in the list diffusingfourth item in the list and reactingfifth item in the list.

     In a simple list, some writers place a comma before the final item; "diffusing, and reacting". You can decide which usage you prefer. A comma before the 'and' for the last item in the list is known as the Oxford comma. It does no harm and is common usage in America. It you leave it out, only a pedant would complain.

   • The physiological processes that control the daily cycle of sleep and wakefirst item in the list, comma separating items in the list hungersecond item in the list, comma separating items in the list activity levelsthird item in the list, comma separating items in the list body temperaturefourth item in the list, comma separating items in the list melatonin level in the bloodfifth item in the list, comma separating items in the list and many other physiological traitssixth (non-specific) item in the list are called the endogenous circadian rhythm.
   
   

   In this list, there is a comma before the final item; ", and many other physiological traits". This Oxford comma is helpful here because the items in the list are mainly phrases rather than single words and it's helpful to know immediately where the penultimate item ends.

2. Comma for marking an adverbial

   Simple adverbs are not usually marked by a comma.

   Each of the different wavelengths is refracted by a slightlythis adverb is not separated by a comma - it's part of the noun phrase "a slightly different amount" different amount.

   Adverbials are different. An adverbial See the glossary definition may be a single word, a phrase, or a clause. It gives different types of information and is not usually a modifier of a single word or phrase. It may provide circumstantial information, indicate the writer's stance, or link one sentence or paragraph to another. Adverbials normally require a comma.

   • Black is the absence of colour – it’s what’s we see when there’s no light at all. On the other handthis is an adverbial signalling contrast, separates the adverbial phrase from the main clause white is a combination of all the colours together.
   • Science, beginning of adverbialin essencethis is an adverbial signalling that what follows is a definition containing only the most important characteristics, end of adverbial involves making and testing factual claims about the world.
   • Brown, beginning of adverbialfor instancethis is an adverbial signalling that what follows is an example, end of adverbial is a mix of red and green.
   • And finallythis is an adverbial signalling that this is the last item in a series (in this case - benefits of hammerheads in sharks), end of adverbial scientists think hammers help sharks make quicker turns while swimming.
   • Not every galaxy looks this way, start of adverbial thoughthis is a linking adverbial signalling that there is something different to come - in this case different forms of galaxies revealed in the sentence which follows.     (context)Most other galaxies that are thin disks similar to our Milky Way also have winding spiral arms. We call these spiral galaxies. Some of the other galaxies we see in the universe look like smooth, fuzzy ovals of light, something between the shape of a basketball and a rugby ball. Not every galaxy looks this way, though. Some of the other galaxies we see in the universe look like smooth, fuzzy ovals of light, something between the shape of a basketball and a rugby ball.



3. Comma for separating adjectives

Adjectives, unless they are separated by "and", may be separated by a comma.

• We call these elliptical galaxies, and they are mostly made up of coolerfirst adjective, comma separating two adjectives reddersecond adjective starsnoun.
• The deeperfirst adjective, comma separating two adjectives hotsecond adjective layersnoun resisted this pulling, creating cracks in the top layer.
• You do slowfirst adjective, comma separating two adjectives gentlesecond adjective, comma separating two adjectives and fluidthird adjective movementsnoun with your body.

The last example has three adjectives having more the character of a list; hence the "and" before the final adjective.




4. Comma for non-restrictive clauses

Non-restrictive clauses are separated from the main clause by commas.

• Levels of the hormone leptinfirst part of main clause, comma separating non-restrictive relative clause which helps control hungernon-restrictive relative clause, comma closing non-restrictive relative clause go upsecond part of main clause.
• As the cluster growsadverbial clause, comma separating adverbial clause from main clause many outside factorsfirst part of main clause, comma separating main clause from non-restrictive ing-clause including humidity and temperaturenon-restrictive ing-clause, comma ending non-restrictive ing-clause begin to affect its overall shapesecond part of main clause.



5. Comma for coordinated independent clauses

Coordinated (linked with a coordinator See the glossary definition) independent clauses See the glossary definition are separated by commas.

• We see a thin line because our galaxy is shaped like a thin diskfirst independent clause, comma ending first independent clause andcoordinator we’re looking at the edge of the disksecond independent clause.
• We call these elliptical galaxiesfirst independent clause, comma ending first independent clause andcoordinator they are mostly made up of cooler, redder starssecond independent clause.
• These are animals that eat plantsfirst independent clause, comma ending first independent clause butcoordinator they’re still considered predatorssecond independent clause.



6. Comma for adverbial prepositional phrases

Adverbial prepositional phrases are separated from main clauses by a comma.

• In Australiaadverbial prepositional clause, comma ending adverbial prepositional clause dingoes are considered apex predatorsmain clause.
• During the nightadverbial prepositional clause, comma ending first adverbial prepositional clause in the darksecond adverbial prepositional clause, comma ending second adverbial prepositional clause body temperature drops, metabolism slows, and the hormone melatonin rises dramaticallythree, comma separated, independent paratactic clauses.
• For exampleadverbial prepositional clause, comma ending first adverbial prepositional clause in my research team’s work studying plant pollen grainssecond adverbial complex prepositional clause, comma ending second adverbial prepositional clause we have seen a huge variety of patternsmain clause, comma ending main clause including spikes, stripes and many morenon-restrictive ing-clause post-modifier.



7. Comma for adverbial phrases

Adverbial phrases are separated from main clauses by a comma.

• When we look up at the night skyadverbial clause, comma ending adverbial clause the stars we can see with our eyes are all part of the Milky Waymain clause.
• As the lava cooledadverbial clause, comma ending adverbial clause the cracks spread deeper and deeper into the rockmain clause.
• So if you must listen to music while you studyadverbial clause, comma ending adverbial clause it may be better to listen to classical pieces rather than the latest pop hitsmain clause.



8. Comma for appositive noun phrases

Noun phrases in apposition are separated from main clauses by a comma.

• Quantum mechanicsfirst part of main clause, comma ending first part of main clause the theory which rules the microworld of atoms and particlesappositive noun phrase, comma ending appositive noun phrase certainly has the X factorsecond part of main clause.
• This mental updating is expressed in a mathematical formula worked out by the 18th-century English scholarmain clause, comma ending main clause Thomas Bayesappositive noun phrase.



9. Comma for reported speech

Reported speech and quoted text are enclosed in quotation marks and separated from the reporting phrase by a comma.

Reported speech is rare in academic texts but quoted text is quite common.

• According to Fisher and Gleitman (2002)reporting (signal) phrase, authors and citation, comma ending reporting phrase ‘quotation mark opening quoted textSix-month-olds use consistent metrical or rhythmic patterns in speech to create coherent word-like unitsquoted text’ quotation mark closing quoted text.

Test your understanding of comma punctuation with the Comma Punctuation Quiz.