The writing system was based on the Old Khmer Script, but it made changes because the Old Khmer Script did not show tones, which was an important part of the Thai language. To make the writing system suitable for the Thai language, the Thai script added new letters and tone markers that would show the tone of a word. It was the first known writing system in the world to show the tones of a word. Even though the Chinese writing system existed before the Thai script was invented and that Chinese was a tonal language at that time, there was no writing system that could show the tones of Chinese words until much later. The first time Chinese would be written using the Latin alphabet and tone markers was the Portuguese-Chinese dictionary written by Matteo Ricci and Lazzaro Cattanero written around the 16th century.