The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 4, 'Abbreviations'
Since abbreviations increase the possibility of confusion and misunderstanding, they should be used with caution. When writing for a particular publication, use only those abbreviations which are likely to be familiar to its readers. Never begin a sentence with an abbreviation, and avoid abbreviations as far as possible in passages of continuous prose. For example:
The author’s comments on page 47, line 20, seem particularly apt.
Here the words ‘page’ and ‘line’, normally abbreviated in references, are given in full to prevent a disruptive effect in reading. Extensively used abbreviations, other than common ones such as ‘p.’ and ‘l.’, should be clearly listed at the beginning of a book or in an early note to an article.