The Large and Medium Case come with 12 compartments, and the Small Case comes with 9 compartments. Detachable system foams sizes correspond to case size. (Large System Foams fit the Large Case, Medium System Foams fit the Medium Case, and the Small System Foams fit the Small Case.Choose your preferred case color from 3 choices.Each compartment comes with independent lids.The Large Case can carry large hackle patterns which is difficult to store in micro slit foam with small fly patterns in one fly case. The Medium Case is suitable for flies smaller than #12.To secure clearance within the case, recommended to use with System Foam for Trout (5mm/0.2inch thickness) available separately.