17 is apparently "the barrier" firstly b/c it comes between 16, relating to a square, and 18, which relates to a rectangle, and secondly because it separates 18 from it's epogdoon 16, the number with which it has the relationship 9:8, the major second (loosely speaking / usually). IDK why the Pythagoreans "abominated" 17; they loved 153, the triangle of 17. (-Perhaps it is partly because of the extremely dissonant nature of the major second interval (or so it was considered until relatively recently, early last century or so)?)
17 is the number of consonants in the classical Greek alphabet; I'm aware of the linking of various entities (elements, stars, planets, etc.) to the letters, but cannot personally evaluate claims of historical religious significance related to isopsephy/gematria - though I'd like to think that the Cambridge School will be vindicated) 😄. ...Perhaps someone with better qualifications can weigh in?
I'm sorry I can't give refs; my notes from that period are sadly incomplete.