X00 Subfield $r - Key for music:

Subfield $r contains the statement of key used in a uniform title for a work in a name/title heading .

Example:

100 1#$aBeethoven, Ludwig van,$d1770-1827.$tSonatas,$mpiano,$nno. 13, op. 27, no. 1,$rE major

Subfield $r is not used for a music key in a title page title used in a name/title heading.

Example:

400 1#$aMozart, Wolfgang Amadeus,$d1756-1791.$tSonata in B flat major, K 292, for bassoon and piano