For more than two authors (in previous editions it was more than three), only the name of the first author is used, followed by et al. (and others). Note that et al. is not italicized in text citations.
If a reference list includes another work of the same date that would also be abbreviated as “Schonen et al.” but whose coauthors are different persons or listed in a different order, the text citations must distinguish between them. In such cases, the first two authors (or as many as needed to distinguish the references) should be cited, followed by et al.
Alternatively, a shortened title, enclosed in commas, may be added. In the following examples, et al. refers to different coauthors, so a, b, and so on cannot be used (see 13.114):
Had the examples above been published in different years, no such adjustments would be necessary (see also 13.124). For treatment of multiple authors in a reference list, see 13.107.