How do you link to a directory one step up from a directory one step down?

For example-

How would I link to:
default.com/a/main.html

From:
default.com/a/b/index.html

Thank you for your time :)

Linking from default.com/a/main.html to default.com/a/b/index.html is <a href="b/index.html"> but seeing as you don't need to link the index.html bit you can simply link it <a href="b">

To go back from default.com/a/b/index.html to default.com/a/main.html would be <a href="/a/main.html">
Putting a / infront means it will go back to the domains start.. So say if your address is http://www.hotpage.com/~user/a/b/index.html you would have to link it like this <a href="/~user/a/index.html>

I am shocking at explaning to i hope you can understand that ^^ :)