Page 155 - Perfect English Grammar: The Indispensable Guide to Excellent Writing and Speaking

Thousands of years ago, there was no punctuation in any of the languages from
which English is derived. But along the way, in order to make the written word

better reflect the spoken word, punctuation, capitalization, and spacing were
introduced to help a reader separate words and ideas from each other, and to

better reflect the natural rhythms that occur in the spoken language.

16.1 Period

Also called the full stop, the period is the way we end sentences in English.
Looking at it another way, it’s also how we separate sentences so they don’t run

into each other. You put a period where a complete idea ends.

Also see section 17.16, Spaces after a Period.
The dot that appears in writing out some numbers, such as dollars and cents,

is not called a period, but a decimal point, or just a point. They look the same

but are treated differently.
For using periods in abbreviations, see section 9.0, Abbreviations.

16.2 Comma

What a useful thing a comma is! It is important to making written English easy

to read, and it has many roles.

You may have learned that commas add breathing room to sentences, so that
your thoughts aren’t all jumbled up. That makes it sound, however, as if you just

throw in a comma whenever you’ve been going on too long. But that’s not the
case. There are general guidelines about using commas that are more about

organizing the parts of our sentences than they are about forcing someone to
pause while reading.

16.2.1 COMMAS AND INDEPENDENT CLAUSES

In general, commas separate independent clauses (see section 5.4, Clauses) when

they are connected by certain coordinating conjunctions (see section 14.1,