The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads.  An isotactic triad (mm) is made up of two adjacent meso diads, a syndiotactic triad  (also spelled syndyotactic) (rr) consists of two adjacent racemo diads, and a heterotactic triad (rm) is composed of a racemo diad adjacent to a meso diad.  The mass fraction of isotactic (mm) triads is a common quantitative measure of tacticity.
When the stereochemistry of a macromolecule is considered to be a Bernoulli process, the triad composition can be calculated from the probability P of a diad being meso. For example, when this probability is 0.25 then the probability of finding:
*an isotactic triad is P, or 0.0625
*an heterotactic triad is 2P(1–P), or 0.375
*a syndiotactic triad is (1–P), or 0.5625
with a total probability of 1. Similar relationships with diads exist for tetrads.