Authors are occasionally required to cite to a reference by page and line number. For example, many adjudicatory boards require briefs to cite to references by paragraph number, if possible, and otherwise by page and line number.
The citation might read: “(Source at 5, lines 10–14)” However, what should it look like when the cited passage ends on a subsequent page? (Source at 5, line 20 through 6, line 2) ?? (Source at 5f., lines 20–2) ?? (Source at p. 5, l. 20–p. 6, l. 2) ?? These all look awful; there must be a better answer.
Does your answer change if the page numbers and line number are similar? Page 20, line 20 through page 22, line 24?
If you are able to cite to a style guide, that is preferred. I tend to favor Chicago Manual of Style ("CMoS"), although almost any commonly used style guide should work. I did not find the answer in CMoS.
Best Answer
If (as you indicate) your standard style for a simple citation takes the form "(Source at 5, lines 10–14)," then it seems to me you are bound to extend that form to more-complicated citations, such as "(Source at 5, line 20, through 6, line 2)." Otherwise, you are, in effect, demanding that readers learn two short citation forms that will appear interchangeably in the book. But the ungainliness of the complicated citation's form isn't really the fault of the complicated citation; it's the fault of the underlying simple form.
One of the short-form text citation examples in the 15th edition of the Chicago Manual of Style (at 16.109) involves multiple page references within a particular source. Here's the relevant section:
Following the "(Barnes 1998, 2:354–55, 3:29)" form above, you could consistently indicate the relevant pages and lines of your citations by adopting the form "(Source, page:line)." In the usual simple case, this would yield short-form citations that look like this:
In the more complicated cases, it would yield entries such as this:
Because a lowercase l looks so much like a 1, I would not add an "l" element to the short form, to avoid making the citations unduly confusing to readers.
The crucial thing in adopting any specialized short-form citation style is to make clear to readers what the elements of the short form are. This is especially true if your short-form style isn't standard at your publisher or in your discipline. The simplest way to introduce such a form is in a note at the beginning of the long-form bibliography, where readers will go in any case when they want to identify a citation in full.