When two authors have written the source, include both their last names in your citation:
(Cullingworth and Nadin 2007)
Cullingworth and Nadin (2007).
When there are three or more authors write the last name of the first author,
followed by et al. whenever you cite the source:
Tayler et al. (2003)
(Tayler et al. 2003)
Sometimes you may find the information you are discussing has been written in several different
sources. In this case cite them all in one set of brackets in chronological order of publication. List any works published in the same year in alphabetical order:
(Midgley 1994; Smith 1994; UNCHS 1996; Gandelsonas 2002).