All manuscripts go through two stages: being submitted for consideration and the delivered and accepted manuscript (the D&A), which is the last version the author approves before it goes to the typesetter.
Formatting Rule #1:
Most of us write without having sold the manuscript. The format for unpurchased manuscripts is up to the author, unless the agent/publisher has specific requirements.
Formatting Rule #2:
The publisher rules. Whatever format or word count method the publisher requires for a delivered and accepted manuscript (the D&A) will follow the rules of the publishing house that accepted it.
Absolute format requirements: these are current industry-wide standards:
8.5” x 11” white bond paper (you might be able to get away with a slight off-white, but it’s tricky)
Black ink: typewriter, with a good ribbon; ink jet, or laser printer
Single-sided/double-spaced
Left justification.
1" to 1.5" margins all around.
A page number on each page, except maybe page 1, and continuous numbering throughout. If Chapter 9 ends on page 120, Chapter 10 begins on page 121, non on 10-1.
A header on each page.
No holes, binding, covers, etc. Ship the manuscript and cover letter in a box just big enough to hold it or in a thick, padded envelope, perhaps with rubber bands around it.