In many condensation polymers, the repeat unit contains two structural units related to the comonomers which have been polymerized. For example, in polyethylene terephthalate (PET or "polyester"), the repeat unit is -CO-CH-CO-O-CH-CH-O-. The polymer is formed by the condensation reaction of the two monomers  terephthalic acid (HOOC-CH-COOH) and ethylene glycol (HO-CH-CH-OH), or their chemical derivatives. The condensation involves loss of water, as an H is lost from each HO- group in the glycol, and an OH from each HOOC- group in the acid. The two structural units in the polymer are then considered to be -CO-CH-CO- and -O-CH-CH-O-.