diff --git a/common/constants.cc b/common/constants.cc
index a9cf238..0677e66 100644
--- a/common/constants.cc
+++ b/common/constants.cc
@@ -112,6 +112,7 @@
 const char kPrefsWallClockStagingWaitPeriod[] =
     "wall-clock-staging-wait-period";
 const char kPrefsManifestBytes[] = "manifest-bytes";
+const char kPrefsPreviousSlot[] = "previous-slot";
 
 // These four fields are generated by scripts/brillo_update_payload.
 const char kPayloadPropertyFileSize[] = "FILE_SIZE";
diff --git a/common/constants.h b/common/constants.h
index 64447ce..68f720d 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -76,6 +76,7 @@
 extern const char kPrefsLastFp[];
 extern const char kPrefsPostInstallSucceeded[];
 extern const char kPrefsPreviousVersion[];
+extern const char kPrefsPreviousSlot[];
 extern const char kPrefsResumedUpdateFailures[];
 extern const char kPrefsRollbackHappened[];
 extern const char kPrefsRollbackVersion[];