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Which Jewish months can vary in length?

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Circular diagram of the Jewish religious year showing Hebrew months around the outer ring and Gregorian months inside, with major holidays and fasts such as Rosh Hashanah, Yom Kippur, Sukkot, Hanukkah, Passover, and Purim labeled with their dates. — Jewish religious year The months and holidays of the Jewish religious year.

In the Jewish calendar, the months of Ḥeshvan and Kislev are the only months that vary in length, as they can each contain either 29 or 30 days depending on the specific year.123

Other months in the Jewish religious year have fixed lengths:
* Full months (30 days): Nisan, Sivan, Av, Tishri, Shevaṭ, and First Adar (in a leap year).3
* Defective months (29 days): Iyyar, Tammuz, Elul, Ṭevet, and Adar (or Second Adar in a leap year).3

Because the calendar is schematic, the variation in the lengths of Ḥeshvan and Kislev determines the total number of days in a given year, resulting in either a "complete," "normal," or "defective" year.3