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Western music is based on a division of 12 distinct frequencies per \textit{octave}. An octave is an \textit{interval}, or distance between two frequencies, that corresponds to a power of 2 multiplication. Musical pitch is perceived in a logarithmic scale---one octave above a given perceived frequency is double that frequency; one octave below is half that frequency. A \textit{semitone} is the smallest interval, equal to $1/12$ of an octave. $n$ semitones above a given frequency $f_0$ can be calculated as $f_0 \cdot 2^{n/12}$.  \textit{Note names} are used to classify these frequencies periodic by octave. Note names correspond to the white keys on a piano---in any one given octave there are the note names $C$, $D$, $E$, $F$, $G$, $A$, and $B$ corresponding to white piano keys (see figure 2.1). ~\ref{fig:piano}).  Each of these base note names can be decorated with an indefinite number of sharps ($\#$) and flats ($b$). Each additional $\#$ increases the The divergence between exactly measurable and calculable to subjectively derived from perception---at the most basic element of music---gives birth to the field of \textit{music informatics retrieval} (MIR), devoted to automatically extracting data and classifying features from works of music.