Canonical Tag usage

Canonical tag is used to keep the url in consistency way in terms of case sensitive (lower case , upper case , proper case of character)
so the google keep the url ranking
for example suppose if we have the url
http://www.blogger.com/Post_Create/example_n.aspx
ok
by using canonical tag
google read as
http://www.blogger.com/post_create/example_n.aspx
canonical tag it read as same page
its ignore the opt=value we add this to rel tag value as in source code