Institutio Oratoria

the letter n, which is required by strict adherence to fact, is forced by the sound of the m. which follows to change into another m.

We must also note when analysing compound words, whether the middle consonant adheres to the preceding syllable or to that which follows. For example since the latter part of haruspex is from spectare, the s must be assigned to the third syllable. In abstemius on the other hand it will go with the first syllable since the word is derived from abstinentia temeti,

abstention from wine.