diff --git a/doc/tex/channelvariables.tex b/doc/tex/channelvariables.tex
index 9d466e5cc62e039b5c35b785646ffdebbf7547f0..4aa78f176f9029deb5751f500a121130e4fd4bd2 100644
--- a/doc/tex/channelvariables.tex
+++ b/doc/tex/channelvariables.tex
@@ -831,6 +831,7 @@ ${QUEUESTATUS}            Status of the call, one of:
 ${RECORDED_FILE}        * Recorded file in record()
 ${TALK_DETECTED}        * Result from talkdetect()
 ${TOUCH_MONITOR}          The filename base to use with Touch Monitor (auto record)
+${TOUCH_MONITOR_PREF}   * The prefix for automonitor recording filenames.
 ${TOUCH_MONITOR_FORMAT}   The audio format to use with Touch Monitor (auto record)
 ${TOUCH_MONITOR_OUTPUT} * Recorded file from Touch Monitor (auto record)
 ${TXTCIDNAME}           * Result of application TXTCIDName