The Comma ( , comma)

How to use the comma correctly

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

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 drying,comma separating items in a list freezing,comma separating items in a list wrinkling,comma separating items in a list diffusing and reacting.
  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. If 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 traits sixth (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.

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 adverbialAn adverb phrase used to provide circumstantial information about a clause, to indicate the writer’s stance, or to link units of discourse by indicating their relationship. 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 adverbial in 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 adverbial for 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 though this 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.

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 redder second adjective stars noun.
• The deeperfirst adjective, comma separating two adjectives hot second adjective layers noun resisted this pulling, creating cracks in the top layer.
• You do slowfirst adjective, comma separating two adjectives gentlesecond adjective, comma separating two adjectives and fluid third adjective movements noun with your body.
  The last example has three adjectives having more the character of a list; hence the "and" before the final adjective.

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 up second 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 shape second part of main clause.

Comma for coordinated independent clauses

Coordinated (linked with a coordinatorA word which connects words, phrases and clauses. A coordinator links words, phrases and clauses which have the same syntactic status (they are not subordinate).) independent clausesA clause which contains a finite verb, can stand alone, and is not part of a larger structure. Independent clauses can be simple (only one clause), complex (an independent clause with one or more dependent clauses, or compound (coordinated dependent clauses). 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 and coordinator we’re looking at the edge of the disk second independent clause.
• We call these elliptical galaxiesfirst independent clause, comma ending first independent clause and coordinator they are mostly made up of cooler, redder stars second independent clause.
• These are animals that eat plantsfirst independent clause, comma ending first independent clause but coordinator they’re still considered predators second independent clause.

Comma for adverbial prepositional phrases

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

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

Comma for adverbial clauses

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 Way main clause.
• As the lava cooledadverbial clause, comma ending adverbial clause the cracks spread deeper and deeper into the rock main 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 hits main clause.

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 factor second 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 Bayes appositive noun phrase.

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 text Six-month-olds use consistent metrical or rhythmic patterns in speech to create coherent word-like units quoted text ’quotation mark closing quoted text.