Size, Diameter, Cup Diameter, Rim Width, Bore Size, Back Bore, Description,4AL,39.44mm,26.00mm, 6.72mm, 7.38mm, Barrel, Classic euphonium model/ good for powerful trombonists5AL,39.00mm,25.74mm, 6.64mm, 7.30mm, Barrel Favorite for symphonic trombone5BS,39.00mm,25.73mm, 6.64mm, 6.87mm, Medium, Gives medium bore trombones a large bore sound 6BS,37.92mm,25.40mm, 6.26mm, 6.66mm, V-Type, All around best seller. Good in all registers