on CMOS Shop Talk:

When you write a book to send to an agent or editor, you are preparing a manuscript. And even if your ideas, characters, and plot twists are colorful and creative, your manuscript format should not be. Agents and editors almost always require submitted pages to be in a standard format free of images and color and anything flashy. Many agents and editors post the format they prefer (or demand) on a “Submissions” page at their website.

It’s normal, however, to prepare material before you know exactly where you’ll submit it. That’s why it’s smart to produce a generic document based on Chicago style (the style used by most US trade book publishers) that can be tweaked later if you receive more specific instructions. Using this conventional style and saving it in a flexible file format will position you to adjust your formatting for each submission without a lot of extra work.

The Chicago Manual of Style doesn’t specifically cover manuscript formats for novels, so these recommendations draw from CMOS 2.7–12, which lay out general best-practice manuscript formatting guidelines for design, production, and typesetting that apply to most types of books, including novels.

Whether agents or editors prefer to receive your manuscript as a mailed printout, in an emailed attachment, or through a submission website,* they will expect to receive it in a format similar to the one described here. These conventions have been around for a long time; in the absence of any contrary instructions, follow them to give your pages a professional look.