Principles of per cola et commata

The practice of dividing text lines by clause and phrase units (per cola et commata) was used for educational purposes by the ancient Romans. St. Jerome, as he says in his prologue, also used this for the benefit of readers in his new translation of the books of the prophets. Units may consist of noun phrases, appositives, adjective phrases, prepositional/ablative phrases, participial phrases, ablative absolutes, subordinate clauses, or items of a parallel list (especially tricola), etc. Smaller grammatical units may be combined to form larger rhetorical units. It is the editor’s choice whether to divide the lines by the smaller units or combine smaller units on one line; I sometimes mark separate smaller units within a line with a line: |.
The system of indentation is not original to the ancient Romans, but my own. Appositives and modifiers (including adjectival relative clauses, participial phrases, etc.) are lined up under their noun-head. Parallel items or clauses are also lined up.
A sentence usually begins flush left. Subordinate clauses or units are indented. Successive parts of a subordinate clause or the main sentence may be lined up or indented, but are never farther left than the beginning of the clause. The resumption of an interrupted clause is lined up where the end of the previous part ended. Subordinate clauses may be lined up under the words that set them up (e.g. result clause under a tam) or lined up with the end of the verb introducing the clause (e.g. verb of saying for indirect statement, verb of commanding for indirect command); if the unit will not fit under indented this way, it is indented farther back, but not to the left of the beginning of the previous clause. Subordinate clauses may be marked by [ ].
A single word is not usually on a separate line unless it is a member of a parallel list or sets up something.
We the people of the United States,
[in order to form a more perfect union,
establish justice,
insure domestic tranquility,
provide for the common defense,
promote the general welfare,
and secure the blessings of liberty
to ourselves and our posterity,]
do ordain
and establish this Constitution
for the United States of America.

For more, see my article: “A Structural Arrangement of Text to Facilitate Reading,” Classical Journal 102.3 (2007) 291-303. (Available on JSTOR).