Musar על מנחות 58:9

Whereas we perceive the full four- lettered name (when spelled according to the גלגול system) to be totally apart from anything developmental, the sequence of the letters י-ה-ו י-ה, י, (what they represent) are all part of the מרכבה, the part of G–d's visible entourage that is represented by the alphabet, i.e. א-ת, plus the five "final" letters ם-ן-ץ-ף-ך, which gives us a total of 27 letters. The letter י represents (10 אבגד). The letters י-ה represent (15 אבגדה), the letters י-ה-ו of the four-lettered name represent (21 אבגדה) of that part of the מרכבה.