diff --git a/wiki/unix/unix.m4 b/wiki/unix/unix.m4
index a68d9c7..0a4df82 100644
--- a/wiki/unix/unix.m4
+++ b/wiki/unix/unix.m4
@@ -16,6 +16,7 @@ _bentr(`_link(`Douglas McIlroy', `https://en.wikipedia.org/wiki/Douglas_McIlroy'
 _bentr(`_link(`Interview with Brian Kernighan', `https://www.linuxjournal.com/article/7035')')
 _bentr(`_link(`Myths about Multics', `https://www.multicians.org/myths.html')')
 _bentr(`_link(`Joe Ossanna', `https://en.wikipedia.org/wiki/Joe_Ossanna') (Wikipedia)')
+_bentr(`_link(`Preliminary Unix Implementation Document - June 1972', `http://www.bitsavers.org/pdf/bellLabs/unix/PreliminaryUnixImplementationDocument_Jun72.pdf')')
 _bentr(`_link(`Ken Thompson', `https://en.wikipedia.org/wiki/Ken_Thompson') (Wikipedia)')
 _bentr(`_link(`Unix History', `https://livinginternet.com/i/iw_unix_dev.htm')')
 _bentr(`_link(`The Unix Oral History Project', `https://web.archive.org/web/20080919055843/http://www.princeton.edu:80/~mike/expotape.htm')')