In Akkadian context, there are basically two ways to “read” a given sign:
(I): a logographic reading; the values are inherited from the Sumerian period. In this case, the sign will be transliterated in capital letters.
(II): a phonographic (or syllabographic) reading; the sign represents the pronunciation of a syllable in either the Sumerian or Akkadian languages. In this case, the (phonemic value of the) sign will be transliterated with small letters.
Logographic signs are, without exception, inherited from Sumerian period (is it correct?). It seems that when being transliterated, their “sign name” is written (such as AN or DU). Presumably, each sign is supposed to have a unique “sign name”. I was wondering where this “sign name” comes from and unfortunately I could not find a straightforward answer. I guess that “sign name” is a Sumerian phonetic reading of a sign. But which reading? We know that cuneiform signs are polyvalent in a Sumerian context and hence there are normally more than a single (Sumerian) phonetic reading for a given sign. Then, for a given cuneiform sign, which of its possible (Sumerian) phonetic readings is crystallized into its sign name?