 The repeat unit or repeating unit is a part of a polymer whose repetition would produce the complete polymer chain except for the AND groups by linking the repeat units together successively along the chain, like the beads of a necklace. The repeat unit is sometimes called the Mer or Mer unit. Mer originates from the Greek word Meros, which means a part. The word polymer derives its meaning from this, which means many Mer's. The repeat unit or Mer is not to be confused with the term monomer, which refers to the small molecule from which a polymer is synthesized. One of the simplest repeat units is that of the addition polymer polyvinyl chloride, CH2CHClN, whose repeat unit is CH2CHCl. In this case the repeat unit has the same atoms as the monomer vinyl chloride CH2CHCl. When the polymer is formed, the C equals C double bond and the monomer is replaced by a C C single bond in the polymer repeat unit, which links by two new bonds to adjoining repeat units. In condensation polymers C examples below the repeat unit contains fewer atoms than the monomer or monomers from which it is formed. The subscript hand denotes a degree of polymerization, that is, the number of units linked together. The molecular mass of the repeat unit, Mr., is simply the sum of the atomic masses of the atoms within the repeat unit. The molecular mass of the chain is just the product hand them are. Other than monotispersed polymers, there is normally a molar mass distribution caused by chains of different lengths. In copolymers there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.