Let alone135,246 is obviously a routine for writing poems, and it is only allowed for the convenience of creation. It is by no means a strong supporter of 135. This sentence has two meanings. First, one or three levels of this sentence can be ignored, but two or four levels must be different; Second, the level from 13 to 35 of the sentence can be the same as this sentence, and the level from 24 to 6 must be the opposite. No matter what conditions, there can be no isolated level and triple level.
There was no such rule before the final version of metrical poems. The predecessors of ancient literature engaged in these poetic rhythms, not to show their talents, but to better show the unique rhythmic beauty of Chinese. Because these measures are the essence of ancient and modern literati.
It doesn't seem to make much sense for modern people to demand these things. Because most of our beginners have very limited vocabulary and skills, we naturally don't want to be bound by meter. Therefore, I often advise young poetry lovers to write unrestrained modern poetry. It's better to rhyme, at least there is no definition of poetry. There are also many modern poems that don't rhyme.
As for what is a flat tone. Since most people born after 1950s have studied Mandarin, I have always suggested using this rhyme to create for the sake of inheriting and developing these cultural treasures. This view may be contrary to that of some experts who advocate using ancient rhymes to make couplets of ancient poems. But it is more suitable for the current situation of young lovers and more conducive to inheriting these cultural treasures.
It is very convenient for modern people to create with this rhyme, as long as they have the level of primary school. Words learned in primary school or in Xinhua dictionary, 1 and 2 are flat, and 3 and 4 are vague. It's that simple. Knowing the level tone, when writing a poem, look for words according to the fixed frame. Many people can't even master modern rhyme, but they seriously engage in ancient rhyme, which is really a waste of time.
In my humble opinion, learning Tang poetry and Song poetry is mainly about learning humanistic spirit and writing skills, rather than showing a higher level because of mastering a few ancient rhymes and traditional Chinese characters.
The above are only personal opinions for your reference. Please correct the fallacy.