There are no hinges to wear out. Windows have been included to enable the teacher to determine if all calculators have been returned. Each Caddy holds 10 calculators (Teacher Kit). The stackable design keeps all the units tied together with a strap (optional purchase). They take up less space and are durable. The convenient handle makes them even more portable. Orders for the Teacher Kits on these pages include this caddy at no additional charge. Caddies may be ordered separately for existing classroom calculators.