Nylon-6,6: TWO different monomers — hexamethylenediamine H2N(CH2)6NH2 (6 carbons) + adipic acid HOOC(CH2)4COOH (6 carbons). The "6,6" means 6 carbons from the diamine + 6 carbons from the diacid. Condensation releases H2O at each amide bond.

Nylon-6: ONE monomer — caprolactam (cyclic amide with 6 carbons). Ring opens under heat → polymerizes. No small molecule lost in ideal ring-opening, but product has same amide linkage as nylon-6,6.

Property comparison: Very similar tensile strength and elasticity. Nylon-6,6 has slightly higher melting point (264 degC vs 215 degC for nylon-6) due to more regular hydrogen bonding between chains.

Both are polyamides — the -CONH- amide linkage is the same as the peptide bond in proteins. This is why nylon was originally described as "synthetic silk."