In the Arabic language, the letters may be ivided into two groups: sun letters (or solar letters) and moon letters (or lunar letters), based on whether or not they assimilate with the ﻝ (l) of a preceding article. The word for "the sun",aš-šams, assimilates, while the word for "the moon", al-qamar, does not, and it was from this circumstance that the two categories of letters were named.