The most commonly used style guide is the Chicago Manual of Style, Chapter 18
from the University of Chicago Press.
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Most clients have me index to the Chicago Manual of Style 15th Edition,
modified by their style preferences / in-house style guide.
Larger publishers usually have their own in-house style guide.
Please forward a copy of the house style sheet and the copyeditor's style
sheet for your book by e-mail along with your Index Quote request.
"It's the little things that make the big things possible.
Only close attention to the fine details of any operation
makes the operation first class." J. Willard Marriott