why use - instead off _ in url - url

Question

why use - instead off _ in url?
Url contain '_' seems like no bad effects.

Answer 1

Underscores are not allowed in a host name. Thus some_place.com is not a valid URL because the host name is not valid. Underscores are permissible in URLS. Thus some-place.com/which_place/ is perfectly legitimate, other concerns aside.
From RFC 1738:
host
[...] Fully qualified domain names take the form as described
in Section 3.5 of RFC 1034 [13] and Section 2.1 of RFC 1123
[5]: a sequence of domain labels separated by ".", each domain
label starting and ending with an alphanumerical character and
possibly also containing "-" characters. The rightmost domain
label will never start with a digit, though, which
syntactically distinguishes all domain names from the IP
addresses.

Answer 2

When you read a_long_sentence_with_many_underscores, because you are reading it by letter or word recognition, your eye tracks along the middle of the line, but when you reach an underscore, your eye is more likely to track down a bit and back up for the next word.
When you read a-long-sentence-with-many-dashes, your eye keeps tracking along the same horizon, and by sight, it is easier for your brain to try and ignore them.
Another good reason is that Google and other search engines rank urls that match to search terms higher when the word separator is a dash.

Answer 3

One main reason is that most anchor tags have text-decoration:underline which effectively hides your underscore.
And, a non-tech savvy user wont automatically assume that there is an underscore :)

Answer 4

By the way... it seems several Java network libraries will not be able to interpret a URL correctly when using underscore:
URI uri = URI.create("http://www.google-plus.com/");
System.out.println(uri.getHost()); // prints www.google-plus.com
URI uri = URI.create("http://www.google_plus.com/");
System.out.println(uri.getHost()); // prints null

Answer 5

It's easier to type (at least on my german keyboard) and see.