Don't \cleardoublepage before mainmatter or backmatter.

dottes.tex

@@ -47,6 +47,15 @@
   \fi}
 \makeatother
 
+% Replace \mainmatter with one that does a \clearpage
+% not a \cleardoublepage.
+\makeatletter
+\renewcommand\mainmatter{%
+   \clearpage
+   \@mainmattertrue
+   \pagenumbering{arabic}}
+\makeatother
+
 % Show the main tune graphic and optional change.
 % Args are:
 %   1. Tune name
@@ -245,8 +254,6 @@ in 2018.
 
 % -----
 
-\cleardoublepage
-
 \chapter{List of tune first lines}
 
 You know the feeling. Everyone has just launched into a well-loved tune.