Molded of autoclavable polypropylene that is heavier than water. Each rack has numbered and lettered rows and the rack end plate has a convenient spot for a label. The racks will stack. The racks are shipped disassembled. Assembly is simple. Should you wish to disassemble the rack to save space, that is also possible. Outside dimensions are 9-3/4" x 4-1/8" x 2-1/2" high.