WHAT COMBINES EQUIPOISE, FLEXIBILITY AND REGULARITY IN EQUAL MEASURE

the iambic pentameter,

 combines equipoise 

flexibility and regularity in equal measure

Iambic pentameter /aɪˈæmbɪk pɛnˈtæmᵻtər/ is a commonly used type of metrical line in traditional English poetry and verse drama. The term describes the rhythm that the words establish in that line, which is measured in small groups of syllables called "feet". The word "iambic" refers to the type of foot that is used, known as the iamb, which in English is an unstressed syllable followed by a stressed syllable. The word "pentameter" indicates that a line has five of these "feet".
Iambic rhythms come relatively naturally in English. Iambic pentameter is the most common meter in English poetry; it is used in many of the major English poetic forms, including blank verse, the heroic couplet, and some of the traditional rhymed stanza forms. William Shakespeare used iambic pentameter in his plays and sonnets.

Simple example

An iambic foot is an unstressed syllable followed by a stressed syllable. The rhythm can be written as:

da DUM

The da-DUM of a human heartbeat is the most common example of this rhythm.
A standard line of iambic pentameter is five iambic feet in a row:

da DUM da DUM da DUM da DUM da DUM

Straightforward examples of this rhythm can be heard in the opening line of William Shakespeare's Sonnet 12:

When I do count the clock that tells the time