diff --git a/README.md b/README.md
index a8fbd52..2a5cfa0 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1299 TILs and counting..._
+_1300 TILs and counting..._
 
 ---
 
@@ -832,6 +832,7 @@ _1299 TILs and counting..._
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)
 - [Order Matters For `rescue_from` Blocks](rails/order-matters-for-rescue-from-blocks.md)
 - [Params Includes Submission Button Info](rails/params-includes-submission-button-info.md)
+- [Params Is A Hash With Indifferent Access](rails/params-is-a-hash-with-indifferent-access.md)
 - [Parse Query Params From A URL](rails/parse-query-params-from-a-url.md)
 - [Perform SQL Explain With ActiveRecord](rails/perform-sql-explain-with-activerecord.md)
 - [Polymorphic Path Helpers](rails/polymorphic-path-helpers.md)
diff --git a/rails/params-is-a-hash-with-indifferent-access.md b/rails/params-is-a-hash-with-indifferent-access.md
new file mode 100644
index 0000000..1efac71
--- /dev/null
+++ b/rails/params-is-a-hash-with-indifferent-access.md
@@ -0,0 +1,32 @@
+# Params Is A Hash With Indifferent Access
+
+When you have an instance of `ActionController::Parameters`—which you often do
+with `params` in a Rails controller—under the hood you have a
+`HashWithIndifferentAccess`.
+
+Here is what the initializer looks like when you call
+`ActionController::Parameters.new(some_hash)`.
+
+```ruby
+def initialize(parameters = {}, logging_context = {})
+  @parameters = parameters.with_indifferent_access
+  @logging_context = logging_context
+  @permitted = self.class.permit_all_parameters
+end
+```
+
+This means you can reference the keys in your parameters as either a _string_
+key or a _symbol_ key.
+
+```ruby
+> params = ActionController::Parameters.new({ username: 'tacocat' })
+=> #<ActionController::Parameters {"username"=>"tacocat"} permitted: false>
+> params['username']
+=> "tacocat"
+> params[:username]
+=> "tacocat"
+```
+
+Note that `ActiveSupport::HashWithIndifferentAccess` is not an ancestor of the
+`ActionController::Parameters` class (like it once used to be), but is just how
+the incoming hash is transformed when initialized.