diff --git a/make-includes/build_lecture_rules.make b/make-includes/build_lecture_rules.make
index 3dd8c9d..5feb900 100755
--- a/make-includes/build_lecture_rules.make
+++ b/make-includes/build_lecture_rules.make
@@ -51,10 +51,10 @@
 #
 # Base file names for the various documents.
 #
-SLIDES?=$(CHAPTER)slides
-FIGURES?=$(CHAPTER)figures
-COMBINED?=$(CHAPTER)combined
-NOTES?=$(CHAPTER)notes
+SLIDES?=$(CHAPTER)_slides
+FIGURES?=$(CHAPTER)_figures
+COMBINED?=$(CHAPTER)_combined
+NOTES?=$(CHAPTER)_notes
 #
 # Standard files to be installed on the web server.
 #