Non-essential vs. essential information

Non-essential material needs punctuation on BOTH sides when placed in the middle.

Example: The moon, Earth’s satellite, affects tides.
You cannot use only the first comma or only the second.

If removing the information leaves a sensible complete sentence, it is non-essential.

Example: My laptop, which I bought last year, is already slow.
Remove it → My laptop is already slow.

Two commas do NOT automatically mean the material between them is non-essential.

You must perform the removal test.

Non-essential material may be enclosed by two commas, two dashes, or two parantheses

Example: The sculpture, created in 1920, was restored.
Example: The sculpture—created in 1920—was restored.
Example: The sculpture (created in 1920) was restored.
The grammar is equivalent; style/emphasis differs.

NEVER mix the pair.
Wrong: The sculpture, created in 1920—was restored.
Use , , OR — — OR ( ).

Do not redundantly put commas around an already parenthetical phrase.

Wrong: The sculpture, (created in 1920), was restored.
Correct: The sculpture (created in 1920) was restored.

A comma CAN follow a closing parenthesis when another rule independently requires the comma.

Example: The sculpture (created in 1920), but not the painting, was restored.
The comma has another grammatical job rather than merely duplicating the parenthesis.

68. A transition used parenthetically in the middle takes two commas.

Example: The experiment, however, succeeded.
Remove however → sentence still works

69. A transition at the end of a sentence normally gets a comma before it.

Example: The experiment succeeded, however.
It still expresses a relationship with the previous sentence.

70. A transition beginning a new clause follows a sentence boundary and takes a comma after it.

Example: The first test failed; however, the second succeeded.

71. End-position transitions connect backward to the previous sentence.

Example: The older model was unreliable. The new model works well, though.
Though contrasts the new-model statement with the one before it.

“That,” names, titles, and “who”

72. A clause beginning with “that” is essential → no comma before or after “that.”
Wrong: The theory, that she proposed was controversial.
Correct: The theory that she proposed was controversial.

73. If optional “that” is deleted, do not replace it with a comma.
Wrong: The theory, she proposed was controversial.
Correct: The theory she proposed was controversial.

74. Rare exception: “that” may be followed by a comma if a separate non-essential insertion begins immediately afterward.
Example: The signals that, according to researchers, appear first are harmless.
The commas belong to according to researchers, not to that.

75. A name/title in the middle of a sentence normally gets either TWO commas or NO commas.
Example: My teacher, Ms. Lee, called. vs. My teacher Ms. Lee called.
Which one is right depends on whether the name is necessary to identify the person.

76. A single comma BEFORE a middle-position name/title is wrong.
Wrong: My friend, Maya arrived.
Either my friend Maya arrived or my friend, Maya, arrived depending on meaning.

77. Name/title essential? Cross it out and check meaning.
Example: The author Toni Morrison won the prize.
If the author alone does not identify whom you mean, the name is essential → no commas.

78. Book shortcut: commas imply the person/thing is uniquely identified; no commas imply one among several.
Example: My sister, Priya, suggests one relevant sister; my sister Priya distinguishes Priya from other sisters.

79. A single comma AFTER an essential name/title can still be correct if another rule requires it.
Example: She published the novel Blue Sky, a story about migration.
The comma introduces the following appositive, not the title itself.

80. “Who” clauses can be essential or non-essential.
Example essential: Students who study regularly improve.
This defines which students → no commas.
Example non-essential: Maya, who studies nightly, improved.
Extra information → commas.