diff --git a/README.md b/README.md
index 879975c..8969a08 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,9 @@
 challenges (`otp-md5`), so this script only constructs MD5-based responses.
 
 # Usage
+The challenge presented by Orthrus can be copy/pasted onto the command line,
+or (absent command line args) into the `Challenge?` prompt.
+
 ```
 [remote]$ sudo bash
 otp-md5 440 someseed ext
@@ -20,14 +23,42 @@
 Response: MEAT SAD JERK STUN ARGO ITS
 NOTE: copied to clipboard
 ```
+or
+`[local]$ ./otp.py otp-md5 440 someseed ext`
+
 Note that you must copy/paste the challenge string from the remote host to the
 prompt on the local host. The response can then be pasted into the remote host's
 `Password:` prompt.
 
-If the local host has the `xclip` program, then the reponse will be automatically
+If the local host has the `xclip` or `pbcopy` program, then the reponse will be automatically
 copied to the clipboard. If that is not present, or does not exit with success,
 then the response string can be manually copied, then pasted to the remote.
 
+The algorithm ("otp-md5") is the default, and the only supported algorithm at
+the moment. On both the command line and the `Challenge?` prompt, that may be
+omitted. For example:
+```
+[local]$ ./otp.py
+Challenge? 440 someseed ext
+Response: MEAT SAD JERK STUN ARGO ITS
+NOTE: copied to clipboard
+```
+or
+```
+[local]$ ./otp.py 440 someseed ext
+Response: MEAT SAD JERK STUN ARGO ITS
+NOTE: copied to clipboard
+```
+
+Note that _some_ Orthus implementations produce extra words (eg. `ext`). These
+will be ignored. The important parts are the algorithm (optional), the sequence,
+and the seed values.
+
+The algorithm associated with a seed will be stored into the `.otp` file,
+and will override any future specification of an algorithm. Or, it shall
+provide the algorithm when it is not explicit on the command line or
+challenge prompt. In all cases, `otp-md5` is the current default.
+
 ## Alternative Usage
 The script examines the program name used to invoke it (`sys.argv[0]`), and
 if that name matches a defined algorithm (`otp-*`), then it will use the command
@@ -56,7 +87,6 @@
 will be used the next time the seed is seen.
 
 # Possible TODO Items
-* If `xclip` fails, try using `pbcopy` to support macOS
 * Use the `keyring` python package to support keyrings instead of a plaintext file
 * Handle MD4 and SHA1 challenges
 * Track RFC 2289 more closely in "acceptable" inputs