diff --git a/SC2018.md b/SC2018.md
index f6c2d84..3e21992 100644
--- a/SC2018.md
+++ b/SC2018.md
@@ -3,11 +3,29 @@
 ### Problematic code:
 
 ```sh
-PLATFORM="$(uname -s | tr 'a-z')"
+PLATFORM="$(uname -s | tr 'A-Z' 'a-z')"
 ```
 
 ### Correct code:
 
 ```sh
-PLATFORM="$(uname -s | tr '[:lower:]')"
-```
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+PLATFORM="$(uname -s | tr '[:upper:]' '[:lower:]')"
+```
+
+### Rationale:
+
+`A-Z` and `a-z` are commonly intended to mean "all uppercase" and "all lowercase letters" respectively. This ignores accented characters in English, and foreign characters in other languages:
+
+    $ tr 'a-z' 'A-Z' <<< "My fiancée ordered a piña colada."
+    MY FIANCéE ORDERED A PIñA COLADA.
+
+Instead, you can use `[:lower:]` and `[:upper:]` to explicitly specify case:
+
+    $ tr '[:lower:]' '[:upper:]' <<< "My fiancée ordered a piña colada."
+    MY FIANCÉE ORDERED A PIÑA COLADA.
+
+### Exceptions:
+
+If you don't want `a-z` to match `é` or `A-Z` to match `Ñ`, you can ignore this message.
+
+Note that the examples used here are multibyte characters in UTF-8. Many implementations (including GNU) fails to deal with them. 
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