Must FixCitationFound in 2-3% of dissertations

Chicago Et Al. Format: The Four-Author Threshold You Need to Know

Found in 2-3% of dissertations using Chicago style. CMOS uses et al. starting at four authors — not three like APA. Getting the threshold wrong shows you're following the wrong style guide.

FIX

For 4+ authors, use first author followed by "et al." with comma before year.

What This Issue Is

Chicago Manual of Style 17th edition uses et al. for works with four or more authors in text citations. For one to three authors, list all names: (Smith, Jones, and Williams, 2020). For four or more, use the first author plus et al.: (Smith et al., 2020). This differs from both APA (three or more) and MLA (three or more).

The period placement is the same across all styles: period after 'al.' only, never after 'et.' The Latin abbreviation 'et al.' stands for 'et alia' (and others). 'Et' is a complete word meaning 'and' and takes no period. 'Al.' is abbreviated and requires one. Getting this wrong — 'et. al' or 'et. al.' — is a formatting error in every style guide.

In the reference list (bibliography), Chicago has its own approach. For works with one to ten authors, list all authors in the bibliography entry. For eleven or more, list the first seven followed by 'et al.' This is more generous than APA's reference list rules. The key takeaway: the in-text et al. threshold (four authors) is different from the bibliography threshold (eleven authors).

Why Your Committee Flags It

Chicago uses "et al." for four or more authors. Incorrect et al. formatting—whether wrong threshold or missing punctuation—undermines your credibility.

Before & After Examples

Before

(Johnson, Smith, Lee, and Park, 2023)

After

(Johnson et al., 2023)

Chicago lists all authors for three-author works — et al. starts at four.

Before

(Smith et al., 2020) — when the work has only three authors

After

(Smith, Jones, and Williams, 2020)

Four authors: use et al. after the first author in Chicago.

Before

(Smith, Jones, Williams, and Lee, 2020)

After

(Smith et al., 2020)

Period after 'al.' only — 'et' takes no period in any style guide.

Before

Smith et. al. (2022) reported increased performance.

After

Smith et al. (2022) reported increased performance.

Self-Check Checklist

Tap each item as you review your chapter.

Related Checks

Must Fix

Citation Format (comma required)

5-10% of dissertations

Should Fix

Ampersand Ban (use "and")

2-4% of dissertations

Should Fix

Number Spelling (0-100)

1-2% of dissertations

Must Fix

Et Al. Format

2-3% of dissertations

Should Fix

Citation Name Consistency

2-5% of dissertations

Frequently Asked Questions

In text citations: use et al. for works with four or more authors. For one to three authors, list all names. In the bibliography: list all authors for works with up to ten authors; for eleven or more, list the first seven followed by et al. The in-text and bibliography thresholds are different.
APA 7th uses et al. starting at three authors. MLA 9th uses et al. starting at three authors. Chicago uses et al. starting at four authors in-text. If you're switching styles, the author-count threshold is one of the most important differences to check.
Yes, in parenthetical citations: (Smith et al., 2020). The comma separates et al. from the year, just like a comma separates a single author name from the year. In narrative citations, the year is in its own parentheses: Smith et al. (2020).

Sources

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