diff --git a/src/event.cpp b/src/event.cpp
index 3fb88b43c..40b281fe5 100644
--- a/src/event.cpp
+++ b/src/event.cpp
@@ -128,6 +128,7 @@ wcstring event_get_desc(const event_t &e) {
             if (e.param1.pid > 0) {
                 result = format_string(_(L"exit handler for process %d"), e.param1.pid);
             } else {
+                // In events, PGIDs are stored as negative PIDs
                 job_t *j = job_get_from_pid(-e.param1.pid);
                 if (j)
                     result = format_string(_(L"exit handler for job %d, '%ls'"), j->job_id,
@@ -209,6 +210,7 @@ static wcstring event_desc_compact(const event_t &event) {
             } else if (event.param1.pid > 0) {
                 res = format_string(L"EVENT_EXIT(pid %d)", event.param1.pid);
             } else {
+                // In events, PGIDs are stored as negative PIDs
                 job_t *j = job_get_from_pid(-event.param1.pid);
                 if (j)
                     res = format_string(L"EVENT_EXIT(jobid %d: \"%ls\")", j->job_id,
diff --git a/src/event.h b/src/event.h
index aa05030d9..f549ce0ec 100644
--- a/src/event.h
+++ b/src/event.h
@@ -53,7 +53,7 @@ struct event_t {
     /// The type-specific parameter. The int types are one of the following:
     ///
     /// signal: Signal number for signal-type events.Use EVENT_ANY_SIGNAL to match any signal
-    /// pid: Process id for process-type events. Use EVENT_ANY_PID to match any pid.
+    /// pid: Process id for process-type events. Use EVENT_ANY_PID to match any pid. (Negative values are used for PGIDs).
     /// job_id: Job id for EVENT_JOB_ID type events
     union {
         int signal;