diff --git a/YubiKey-PIV-Card.md b/YubiKey-PIV-Card.md
index 1459c19..73dfac8 100644
--- a/YubiKey-PIV-Card.md
+++ b/YubiKey-PIV-Card.md
@@ -16,4 +16,8 @@ Authenticates against physical access applications (e.g. door locks). By default
 
 #### Slot f9: Attestation
 
-Attests other slot keys were generated on the device.
\ No newline at end of file
+Attests other slot keys were generated on the device.
+
+
+#### Reference
+https://ruimarinho.gitbooks.io/yubikey-handbook/content/piv/use-cases.html