It seems that the first written mention of ''Serdica'' was made during his reign and the last mention was in the 19th century in a Bulgarian text (Сардакіи, ''Sardaki''). Other names given to Sofia, such as ''Serdonpolis'' (Σερδών πόλις, "City of the Serdi" in Greek) and ''Triaditza'' (Τριάδιτζα, "Trinity" in Greek), were mentioned by Byzantine Greek sources or coins. The Slavic name ''Sredets'' (Срѣдецъ), which is related to "middle" (среда, "sreda") and to the city's earliest name, first appeared on paper in an 11th-century text. The city was called ''Atralisa'' by the Arab traveller Idrisi and ''Strelisa'', ''Stralitsa'' or ''Stralitsion'' by the Crusaders.