Two Basic Weights

Derived Nouns: Noun of Place and Time (Ism Dharf)

The two patterns are chosen from the Mudhaari' vowel of the middle root letter.

Ism Dharf weightMudhaari' middle vowel
مَفْعَلٌDhammah or Fathah
مَفْعِلٌKasrah

So the Mudhaari' helps you choose which of the two weights to use.