I know this is perfectly reasonable abbreviation, but each time I see it I start trying to work out which specific set of gauges this might be, a confusion born of Rotosound's quirky choices of codes and names for different sets:SB66
RS66LE Heavy 50-70-85-110
RS66LF Custom 45-65-85-105
RS66LD Standard 45-65-80-105
SM66 Hybrid 40-60-80-100
RS66LC Medium 40-60-75-95
RS66LB Medium Light 35-55-70-90
FM66 Funkmaster 30-50-70-90
RS66LA Extra Light 30-50-65-85