por h o por b

Etymology
Presumably, as a general excuse when learning (Spanish) orthography, since the use of the letter B is mostly etymological, and not intuitive at all (bearing in mind that, in most Spanish-speaking regions, the sound of b and v is exactly the same), and the use of h is even more arbitrary, since the letter h is completely silent and its use is pure convention.

Adverb

 * 1)  for one reason or another