Google interprets dashes (-) in phrases as word separators, but interprets underscores (_) as part of a single word. Therefore the URL small_business_website.html is seen as a single word, and small-business-website.html is separated as 3 words “small”, “business” and “website”.
To Google:
“small-business-website” = the words “small” and “business” and “website”, but
“small_business_website” = the word “small_business_website”.
Why does this matter? Because Google considers the words in a URL when calculating a best match. Nobody will ever search for the word “small_business_website”.
This is good:
small-business-website.html
These are bad:
small_business_website.html
smallbusinesswebsite.html
