Two adjacent structural units in a polymer molecule constitute a diad. Diads overlap: each structural unit is considered part of two diads, one diad with each neighbor. If a diad consists of two identically oriented units, the diad is called a meso diad (abbreviated m) as in a meso compound.  If a diad consists of units oriented in opposition, the diad is called a racemo diad (r) as in a racemic compound.  In the case of vinyl polymer molecules, a meso diad is one in which the substituents are oriented on the same side of the polymer backbone: in the Natta projection they both point into the plane, or both point out of the plane.