The old contracts from the pre-twentieth century Taiwan form an enormous corpus of placenames for studying the linguistically internal and external mechanisms to regulate the written forms of placenames. This paper tries to explain, when an authoritative orthography was absent, how the written forms of some placenames kept stable, but some did not. This study extracted representative cases from these old contracts and classifies these placenames according to their etymological origin (Sinitic or Austronesian) as well as their stability in written forms. The result shows that knowing the etymology of a placename helped the writer to pick correct characters for the placename upon writing, resulting in the uniformity of its written form. I would call this mechanism ＂semantic calibration＂. This mechanism served as the semiotic interface to introduce the conventional association between character and meaning to regulate the written form of placename. This proposal is supported by the routine practice of instructing someone to write correct characters in the culture using Chinese characters. Moreover, ＂semantic calibration＂ also stimulated folk etymologies of placename to explain the written form which they observed, and it is called ＂rationalized etymology＂ in this paper. Beside the internal mechanism, the other regulatory force came externally from the stamps held by local headmen. The scribes of contracts could refer to these stamps to find the correct characters of the placename in question, and these stamps successfully regulated the written form of the Austronesian-origin placenames. However, if the written forms inscribed on the stamps varied, a stable form is not expectable because the scribes had no standard to follow.