Tacticity refers to the relative stereochemistry of adjacent chiral centers within a polymer chain.

The practical significance of tacticity rests in the link between tacticity and the physical properties of the polymer.

An atactic polymer has no regularity from chiral center to chiral center. Polymers that are formed by free-radical mechanisms are usually atactic.

A syntactic (or syndiotactic) polymer has successive chiral centers with alternating stereochemical configurations. This is rare.

An isotactic polymer has each successive chiral center in the same stereochemical configuration. This is the result of, e,g, forming polypropylene by Ziegler-Natta catalysis

The regularity of the polymer's structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder.