diff --git a/src/main/java/us/fatehi/creditcardnumber/AccountNumberComplete.java b/src/main/java/us/fatehi/creditcardnumber/AccountNumberComplete.java
index 4f876b8..8e98879 100644
--- a/src/main/java/us/fatehi/creditcardnumber/AccountNumberComplete.java
+++ b/src/main/java/us/fatehi/creditcardnumber/AccountNumberComplete.java
@@ -20,6 +20,7 @@ final class AccountNumberComplete extends BaseRawData implements AccountNumber {
 
   private static final long serialVersionUID = -7012531091389412459L;
 
+  // See https://www.iso.org/news/2016/11/Ref2146.html
   private static final int IIN_LEN = 8;
 
   private final DisposableStringData accountNumber;