What do you mean by "Do they affect SEO?"
They are recommended to be used for the URL over other symbols. Instead of a space bar, the underscore is used to tell search eingnies that it is 2 different work.
To answer your question, it doesn't affect seo directly. It won't harm your seo if you use it.
Are you talking about their use in SEO URLs? If so then use hyphens as Google treats a hyphen as a word separator, but treats an underscore as a word joiner. For example, my-own-blog is "my own blog" to Google, and can be optimized as such for "my own blog" keyword. But "my_own_blog" is the same as "myownblog" and I doubt it that you'd be optimizing your site for such words. Google has confirmed this many times themselves that you should use hyphens or dashes, as put by Google's very own 'Matt Cutts', is the way to go.
Hal Smith
URLdreamer Consultant
If your are using for separate keywords then it will affect it will considered only one keyword if you use underscore always use commas for the separation of keywords.
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Do they affect SEO?
