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.