What are the pros and cons of moving one site onto a subdomain of another site?
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Two sites. One has weaker sales. What would the benefits and problems for SEO of moving the weak site from its own domain to a subdomain of the stronger site?
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I would be more inclined to move it to a subdirectory on the stronger site.
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So it could get the full benefit of the stronger site's domain authority?
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Aye.

Oh, and hey! I just noticed I know you from NOLA.

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Hey! That sure was a great seminar.
That advice makes sense. Thanks.
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Cons:
May lose some of the link juice from the 301 redirect from the old URL's (between 1 and 10% is the usual quoted figure)
Initial (usually short term) drop in rankings and potentially traffic of weaker site pages
www.weakersite.com is a "nicer" looking URL for offline marketing than www.strongersite.com/weakersite
Pro's:-
If on Subfolder rather than sub directory pages will benefit from domain authority and links to domain of stronger site.
Can focus future link building and on-page SEO efforts on one site rather than 2, potentially doubling the amount you can get done

If building from scratch and choice of 2 sites or 1 I would say go for one. As two already exist there is more of a call to be made.