The Comma ( , )

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, freezing, wrinkling, 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 wake, hunger, activity levels, body temperature, melatonin level in the blood, and many other physiological traits 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 adverbial 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 hand, white is a combination of all the colours together.
• Science, in essence, involves making and testing factual claims about the world.
• Brown, for instance, is a mix of red and green.
• And finally, scientists think hammers help sharks make quicker turns while swimming.
• Not every galaxy looks this way, though .     (context)

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 cooler, redder stars .
• The deeper, hot layersnoun resisted this pulling, creating cracks in the top layer.
• You do slow, gentle, and fluid movements 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 leptin, which helps control hunger, go up .
• As the cluster grows, many outside factors, including humidity and temperature, begin to affect its overall shape .

Comma for coordinated independent clauses

Coordinated (linked with a coordinator) independent clauses are separated by commas.

• We see a thin line because our galaxy is shaped like a thin disk, and we’re looking at the edge of the disk .
• We call these elliptical galaxies, and they are mostly made up of cooler, redder stars .
• These are animals that eat plants, but they’re still considered predators .

Comma for adverbial prepositional phrases

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

• In Australia, dingoes are considered apex predators .
• During the night, in the dark, body temperature drops, metabolism slows, and the hormone melatonin rises dramatically .
• For example, in my research team’s work studying plant pollen grains, we have seen a huge variety of patterns, including spikes, stripes and many more .

Comma for adverbial clauses

Adverbial phrases are separated from main clauses by a comma.

• When we look up at the night sky, the stars we can see with our eyes are all part of the Milky Way .
• As the lava cooled, the cracks spread deeper and deeper into the rock .
• So if you must listen to music while you study, it may be better to listen to classical pieces rather than the latest pop hits .

Comma for appositive noun phrases

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

• Quantum mechanics, the theory which rules the microworld of atoms and particles, certainly has the X factor .
• This mental updating is expressed in a mathematical formula worked out by the 18th-century English scholar, Thomas Bayes .

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), ‘ Six-month-olds use consistent metrical or rhythmic patterns in speech to create coherent word-like units ’.

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