grammarians attempt to give systematic descriptions of the sounds of the language. In other authors one can sometimes find occasional remarks about correct pronunciation of certain sounds. However, both types of evidence are often difficult to interpret, because the phonetic terminology of the time was often vague, and it is often not clear in what relation the described forms of the language stand to those which were actually spoken by different groups of the population. Important ancient authors include: Dionysius Thrax Dionysius of Halicarnassus Aelius Herodianus Cross-dialectal comparison Sometimes the comparison of standard Attic Greek with the written forms of other Greek dialects, or the humorous renderings of dialectal speech in Attic theatrical works, can provide hints as to the phonetic value of certain spellings. An example of this treatment with Spartan Greek is given above. Loanwords The spelling of Greek loanwords in other languages and vice versa can provide important hints about pronunciation. However, the evidence is often difficult to interpret or indecisive. The sounds of loanwords are often not taken over identically into the receiving language. Where the receiving language lacks a sound that corresponds exactly to that of the source language, sounds are usually mapped to some other, similar sound. In this regard, Latin is of great value to the reconstruction of ancient Greek phonology because of its close proximity to the Greek world which caused numerous Greek words to be borrowed by the Romans. At first, Greek loanwords denoting technical terms or proper names which contained the letter were imported in Latin with the spelling or , indicating an effort to imitate, albeit imperfectly, a sound that Latin lacked. Later on, in the 1st centuries AD, spellings with start to appear in such loanwords, signaling the onset of the fricative pronunciation of . Thus, in the 2nd century AD, replaces . At about the same time, the letter also begins to be used as a substitute for the letter , for lack of a better choice, indicating that the sound of Greek theta had become a fricative as well. For the purpose of borrowing certain other Greek words, the Romans added the letters and to the Latin alphabet, taken directly from the Greek one. These additions are important as they show that the Romans had no symbols to represent the sounds of the letters and in Greek, which means that in these cases no known sound