diff --git a/actionpack/lib/action_dispatch/routing/url_for.rb b/actionpack/lib/action_dispatch/routing/url_for.rb
index 6c3fc5126a9c7c61154111f702fd630b4fc7671b..d4db78a25ae2bd3d10a668a4b1fa7692ce266608 100644
--- a/actionpack/lib/action_dispatch/routing/url_for.rb
+++ b/actionpack/lib/action_dispatch/routing/url_for.rb
@@ -119,8 +119,9 @@ def url_options
# to split the domain from the host.
# * :domain - Specifies the domain of the link, using the +tld_length+
# to split the subdomain from the host.
- # * :tld_length - Optionally specify the tld length (only used if :subdomain
- # or :domain are supplied).
+ # * :tld_length - Number of labels the TLD id composed of, only used if
+ # :subdomain or :domain are supplied. Defaults to
+ # ActionDispatch::Http::URL.tld_length, which in turn defaults to 1.
# * :port - Optionally specify the port to connect to.
# * :anchor - An anchor name to be appended to the path.
# * :trailing_slash - If true, adds a trailing slash, as in "/archive/2009/"