If I am citing a commonly studied play that does not use line numbers, should I cite act and scene only?
Yes. And if you think it would help your readers, you may also provide the page number in your in-text citation. You may also want to provide a note that explains how you are citing the play. In an essay on Macbeth, you could write the following:
In Macbeth, Shakespeare often returns to the idea that appearances can be deceiving. After hearing of the death of the traitorous Thane of Cawdor, who had seemed so loyal, Duncan remarks, “There’s no art, / To find the mind’s construction in the face” (54; 1.4).1
1. The edition of Macbeth I used does not provide line numbers. I cite the play by page number followed by act and scene numbers.
Shakespeare, William. Macbeth. The Plays of William Shakespeare, vol. 10, edited by George Seeves et al., T. Davison, 1813, pp. 11–295.
Published 17 March 2020