diff --git a/lectures/L21.tex b/lectures/L21.tex
index 2246478..7e96e42 100644
--- a/lectures/L21.tex
+++ b/lectures/L21.tex
@@ -205,7 +205,7 @@ \subsection*{Atomic Types}
 \end{lstlisting}
 
 
-The following functions are used to swap two values, only if the old value matches the expected (i.e., what was provided as the second argument):
+The following functions are used to swap two values, that is, if the current value of *ptr is oldval, then write newval into *ptr:
 
 \begin{lstlisting}[language=C]
 bool __sync_bool_compare_and_swap( type *ptr, type oldval, type newval );
@@ -282,4 +282,4 @@ \subsection*{Atomic Types}
 
 \input{bibliography.tex}
 
-\end{document}
\ No newline at end of file
+\end{document}