What is the meaning of the two parallel lines in Masonic symbolism?

In Craft symbolism, the two parallel lines appear bordering the point within a circle, though the symbol is not found in the earliest Masonic instructions and does not appear in lectures prior to William Preston, who applied it to the two Saints John 2.

The symbol is most commonly explained through the two Saints John, both in astronomical and historical terms:

Astronomically, monitorial lectures treat the lines as representing the feast days of Saint John the Baptist and Saint John the Evangelist, marking the summer and winter solstices 2. In this sense, the lines serve as a calendar boundary, denoting the fixed limits of the sun's circuit through the year 2.

Historically and doctrine-wise, an eighteenth-century lecture explains that Saint John the Baptist drew the first parallel line of the Gospel through his preaching of repentance, while Saint John the Evangelist drew the second line parallel to it by completing with learning what the Baptist had begun with zeal 2.

Other commentators and lectures offer further symbolic readings:

Walter Leslie Wilmshurst interprets the lines as bounding the circle of individual human existence, with one line representing Moses (law and divine ordinances) and the other representing King Solomon (wisdom and divine mercy) 1.

Charles T. McClenachan notes an ancient Egyptian precursor featuring a circle bounded by two parallel serpents representing the Divine attributes of Power and Wisdom 2. He also records a ritual suggestion that the lines recall the brethren standing in two parallel lines while the initiate circumambulates the altar 2.