which is better to add two names (-) or(_)

hi when i write css or html i found that i want add t开发者_开发技巧wo name like this

web-development
web_development

which one is better according SEO or write style name, file name or image name.

The first one is better. Also see this post by Google employee Matt Cutts: http://www.mattcutts.com/blog/dashes-vs-underscores/

use the dash. Google engines don't really parse underscores. This is maybe for programmers sanity, so that when they search for query_function, they get results they are looking for?

If you have a url like "http://example.com/web-site", google will return results for 'web', 'site' and '"web site"'. This is not the case for underscores: web_site will only return results for web_site.

ps. I also think that dashes are better than underscores for usability purposes: a dash is a single button on the keyboard, while an underscore requires two buttons to be pressed. This has nothing to do with the technical side of SEO, but everything to do with usability, which is more important than SEO imo.

for css i don't think there is some issues with naming methodology, but for naming HTML pages - is preferred as search engines take - as space, even though good page name is not enough for good s.e.o. you need to have proper meta tag and keywords.

And make sure all your images have proper title tag, this is real essential.

Isn't it common practice to use the - to connect two words, and the _ to replace a space in situations where you can't use a space/+ sign, like CSS classNames?

first one is better in terms of SEO. Because the priority of hiphen is greater than under score

1. Please list two (2) words in the English language that use underscores ("_") within them.
2. Now list fifty (50) words that use dashes/hyphens ("-").

My opinion is that the hyphens would be a better solution for SEO.

IMO When it comes down to SEO is that everything makes a difference !

You are dealing with two different problems: URLs and CSS. For URLs, hyphens would be the better choice because of SEO.

However, depending on your editing program, underscores might work better for mutli-word class names. In TextMate for instance, I can hit Esc to finish (auto-complete) a class I previously entered. It stops completing when it encounters a hyphen, but will fill in the whole class name when you use an underscore. If this is not the case for your editor, then it is really up to your preference.