+Though not strictly necessary, @emph{bar checks} should be
+used in the input code to show where bar lines are expected
+to fall. They are entered using the bar symbol, @code{|}.
+With bar checks, the program can verify that you've entered
+durations that make each measure add up to the correct
+length. Bar checks also make your input code easier to
+read, since they help to keep things organized.
+
+@c This example deliberately produces a warning
+@lilypond[verbatim,quote]
+\relative {
+ g'1 | e1 | c2. c' | g4 c g e | c4 r r2 |
+}
+@end lilypond
+
+If you compile the code in the example above, you should see a
+warning in the console output:
+
+@example
+warning: barcheck failed at: 1/2
+ g'1 | e1 | c2. c'
+ | g4 c g e | c4 r r2 |
+@end example
+
+Although the missing duration is clear in the musical output in this
+simple example, the warning in the console output is far more
+effective in drawing attention to the missing @code{4} in bar 3.
+
+@seealso
+Notation Reference:
+@ruser{Bar and bar number checks}.
+
+
+@node Pitches and key signatures
+@subsection Pitches and key signatures
+
+@menu
+* Pitch alterations::
+* Key signatures::
+* Warning key signatures and pitches::
+@end menu
+
+@warning{New users often misunderstand how LilyPond uses the key
+signature -- please read the warning at the bottom of this page.}
+
+@node Pitch alterations
+@unnumberedsubsubsec Pitch alterations
+