A triangular number and a repdigit, sixty-six has the tidy form that number-mystics enjoy, but it carries no traditional religious symbolism beyond the biblical book-count; the doubled six sometimes invites uneasy comparison with six hundred and sixty-six, though the two are unrelated in scripture. In the twentieth century the number gained an incidental fame through Route 66, the American highway, a purely modern association. Numerology reduces it to three and reads it through that root. The honest core of sixty-six is the Protestant canon, a fact of ecclesiastical history rather than of number lore.
Florence Campbell, a student of Balliett's, restated and extended the chart in Your Days Are Numbered in 1931, and Juno Jordan, founder of the California Institute of Numerical Research, standardised the modern four-part chart of life path, expression, soul urge and personality numbers in 1965; both writers worked from name and birth date rather than from any older number philosophy.
The genuine Pythagorean number philosophy that this modern chart borrows its name from left no writings of its own: Pythagoras and his early followers taught only by mouth, and what survives of their number theory comes instead through Nicomachus of Gerasa, whose Introduction to Arithmetic of the first century CE became the standard authority for a thousand years, and through Iamblichus, who drew on Nicomachus two centuries later; neither wrote anything resembling a name-and-birthdate numerology.