Thermosetting polymers have a high degree of cross-linking and three dimensional co-valently bonded structures can be formed if at least three covalent bonds link the functional groups used to build the structure. The condensation reaction between two phenol molecules and formaldehyde is illustrated below. Water is the condensation product. In the reaction a carbon-carbon bond joins the phenols. This reaction may also take place at some of the other sites normally occupied by hydrogen atoms.

From: Askeland, "The Science of Engineering Materials," PWS (1994)

Due to mechanical interference between the CH₂ groups, only three hydrogen sites may react with the formaldehyde, and the phenol molecule is said to be 'tri-functional.' This tri-functionality permits the formation of a three dimensional structure in the phenolic. Phenolics are used as adhesives, in coatings, as a composite material matrix, and for electrical insulators.