How to choose the cross connector for greenhouse?

The Cross Joint Purlin Clamp is a high quality, durable piece of hardware designed specifically for greenhouse construction. Available in two materials, galvanized steel and aluminum, it is used to securely connect and support purlins, the important horizontal beams that form the roof and side frames of a greenhouse.

#Hoop Size: 1-3/8”[1.315" O.D.], 1-5/8"[1.66" O.D.] 2”[1.90”O.D.], 2-1/2”[2.375”O.D.]
#Purlin Size: 1-3/8" [1.315"O.D.], 1-5/8”[1.66"O.D.]
#Material: Galvanized Steel Or Aluminum
#Each set includes two Clamps with two Bolts and Nuts