Writing is an art, not a science, so good writers differ on many matters. The Oxford Comma is one example.

For those who may not be aware, the Oxford Comma is used after the next-to-last item in a series of three or more. In the following sentence the Oxford Comma is the one after rain.

The weather forecast calls for high winds, rain, and possible hail.

Some, but not all stylebooks require the Oxford Comma. The Oxford University Press style guidelines, from which it gets its name, recommends it. However, the Associated Press Stylebook discourages it in most circumstances. The issue won’t be resolved in our lifetimes, so what’s a writer to do? Use it or don’t use it, whichever you prefer. But whatever your choice, it’s important to always be consistent.

In a similar way, sentences and paragraphs must be consistent within themselves. There must be no changes in tense, voice, person, number, or parallel structure.

All of the following sentences or paragraphs are poorly written; see if you can spot the errors.

1. Betty arrived at the airport early, but then she realizes she forgot her purse.

2. Mark got down on one knee. Then I was asked to be his wife.

3. Police are asking for the public’s help. If you have any information call 814-555-1212.

4. Major league baseball players earn large salaries, although a minor league player doesn’t.

5. Florida residents can go fishing, sailing, or find a place to hike without driving far.

Here are explanations of why these sentences aren’t consistent, along with rewritten, consistent versions.

1. In “Betty arrived at the airport early, but then she realizes she forgot her purse” Betty arrived is past tense; she realizes is present tense. Rewrite the sentence as:

Betty arrived at the airport early, but then she realized she forgot her purse.

2. For the sentences, “Mark got down on one knee. Then I was asked to be his wife” the issue is that the first sentence is active voice; the second is passive voice. It should be:

Mark got down on one knee. Then he asked me to be his wife.

3. In “Police are asking for the public’s help. If you have any information call 814-555-1212” the problem is a switch between third person and second. Remain consistent by writing:

Police are asking for the public’s help. Anyone with information should call 814-555-1212.

4. For “Major league baseball players earn large salaries, although a minor league player doesn’t” the issue is a switch between plural and singular. A better version is:

Major league baseball players earn large salaries, although minor league players don’t.

5. “Florida residents can go fishing, sailing, or find a place to hike without driving far” violates parallel structure. Note the difference between fishing, sailing, and find a place to hike. A consistent, parallel version is:

Florida residents can go fishing, sailing, or hiking without driving far.

Your readers may not care if you write in past or present tense, or second or third person. But they’ll care if you change tenses in mid-sentence, or person in mid-paragraph. Maintain consistency.