+Key signatures together with the pitch (including alterations) are
+used together to determine when to print accidentals. This is a
+feature that often causes confusion to newcomers, so let us explain it
+in more detail:
+
+
+LilyPond has a sharp distinction between musical content and
+layout. The alteration (flat, natural or sharp) of a note is part of
+the pitch, and is therefore musical content. Whether an accidental (a
+flat, natural or sharp @emph{sign}) is a printed in front of the
+corresponding note is a question of layout. Layout is something that
+follows rules, so accidentals are printed automatically according to
+those rules. The pitches in your music are works of art, so they will
+not be added automatically, and you must enter what you want to hear.
+
+For example, in this example:
+@lilypond[fragment]
+\set Staff.TimeSignature = \turnOff
+\key d \major
+d' cis' fis'
+@end lilypond
+
+@noindent
+no note gets an explicit accidental, but still you enter
+
+@example
+\key d \major
+d cis fis
+@end example
+
+The code @code{d} does not mean ``print a black dot just below the
+staff.'' Rather, it means: ``a note with pitch D-natural.'' In the key
+of A-flat, it gets an accidental:
+
+@lilypond[fragment]
+\set Staff.TimeSignature = \turnOff
+\key as \major
+d'
+@end lilypond
+
+@noindent
+@example
+\key as \major
+d
+@end example
+
+Adding all alterations explicitly might require some more effort when
+typing, but the advantage is that transposing is easier, and music can
+be printed according to different conventions. See @ref{Accidentals}
+for some examples how accidentals can be printed according to
+different rules.
+
+