phonosemantic principles and follow the rule of thumb to "if there is a side, read the side" (, yǒu biān dú biān) and take one component to be a phonetic, which often results in errors. Since the sound changes that had taken place over the two to three thousand years since the Old Chinese period have been extensive, in some instances, the phonosemantic natures of some compound characters have been obliterated, with the phonetic component providing no useful phonetic information at all in the modern language. For instance, (yú, /y³⁵/, 'exceed'), (shū, /ʂu⁵⁵/, 'lose; donate'), (tōu, /tʰoʊ̯⁵⁵/, 'steal; get by') share the phonetic (yú, /y³⁵/, 'a surname; agree') but their pronunciations bear no resemblance to each other in Standard Mandarin or in any modern dialect. In Old Chinese, the phonetic has the reconstructed pronunciation *lo, while the phonosemantic compounds listed above have been reconstructed as *lo, *l̥o, and *l̥ˤo, respectively. Nonetheless, all characters containing are pronounced in Standard Mandarin as various tonal variants of yu, shu, tou, and the closely related you and zhu. Simplification Since the phonetic elements of many characters no longer accurately represent their pronunciations, when the People's Republic of China simplified characters, they often substituted a phonetic that was not only simpler to write, but more accurate for a modern reading in Mandarin as well. This has sometimes resulted in forms which are less phonetic than the original ones in varieties of Chinese other than Mandarin. (Note for the example that many determinatives were simplified as well, usually by standardizing cursive forms.) Derivative cognates The derivative cognate () is the smallest category and also the least understood. In the postface to the Shuowen Jiezi, Xu Shen gave as an example the characters kǎo "to verify" and lǎo "old", which had similar Old Chinese pronunciations (*khuʔ and *C-ruʔ respectively) and may have had the same etymological root, meaning "elderly person", but became lexicalized into two separate words. The term does not appear in the body of the dictionary, and may have been included in the postface out of deference to Liu Xin. It is often omitted from modern systems. Modern classifications The liùshū had been the standard classification scheme for Chinese characters since Xu Shen's time. Generations of scholars modified it without challenging the basic concepts. Tang Lan () (1902–1979) was the first to dismiss liùshū, offering his own sānshū (), namely xiàngxíng (), xiàngyì () and