I understand that 两 is used instead of 二 when dealing with quantities of two. However, my question is - why? What are the historical reasons behind this?
I looked at Wikitionary but it only had historical information for when 两 is used to mean the "tael" unit － not for its definition of "two."
The historical reason behind it is kind of surprising though. From a Chinese Characters Roots book:
The earlier form [of the character] looked like a yoke and a pair of saddles of a two-horse carriage. The initial meaning was two or double. It was also a unit to count vehicles and written as 辆 later.
That is why we have:
Where you notice in 辆 that there is the 车 character, meaning car, at the left side of the 两 character.
I assume its usage became a practical one, since as in many languages (including English), you can be more specific when you are referring to a pair of objects by using 两, instead of two objects using 二.
Here's a particular example: 二炮 (Èr pào) refers to the Second Artillery and 两炮 (liǎng pào) means two guns. So, would you request two guns from your commander or a full regiment?
This might be what you are looking for:
http://xh.5156edu.com/html3/1700.html
两, or 兩:
双。用于鞋娄〖two〗
一两棕鞋八尺藤,广陵行遍又金陵。——唐·戴叔伦《忆原上人》
It's used as "pair" along with describing shoes. Noted that this is more a usage than the origin of the word.
In ancient received texts such as the Shījīng, the Yìjīng, and others, the primary attested meaning of liǎng 兩 is 'two'. This is specifically in reference to paired sets of things, not just any two things taken together.
In the Hàn the word begins to appear regularly as a general word for "two" or "a couple of".
As a measure word, it is used in ancient texts not only with two-wheeled carriages, but also with wooden clogs (normally two to a set) and lengths of cloth (normally two zhàng 丈 apiece). We read the measure word liàng, but there are ancient scholia that say there is no need to distinguish this word from "two" in pronunciation. A measure word is fundamentally a noun, and there are cases of nouns being read in the qùshēng — very roughly speaking, the Mandarin fourth tone.
The usage as a measure word gave rise to the sense "tael", because a tael is 24 zhū 銖; the huángzhōng 黃鍾 'type of ceremonial bell' weighed 12 zhū 銖, so twice that amount was called a liǎng "two[-fold]". So "tael" is really a derived meaning of the basic word.
I am not a native speaker, but I am answering, because I feel the previous answers explain the development of the character 兩 and not why it become popular. Native speakers please correct me. My knowledge is very limited.
@Armfoot has already explained the advantage of having different words for "pair" and ordinal "two". It remains to show why 兩 replaced 二 even in cases where it does not mean pair. From watching 武俠連續劇 it appears that in the past expressions like
二位姑娘
were the norm.
My naive hypothesis
With the development of vernacular Chinese 個 become the most common measure word. People felt that 兩個 was smoother to articulate than 二個。
To be absolutely clear, even for me it is trivial to say 二個，二個，二個。But 兩個 seems smoother. Just like the 一 in 一個 shifts to second tone for better flow.
Great question!
From a calligraphic perspective, the character 两 represents two people (人人) attached to some kind of vehicle or container. Thus it depicts a more accurate description of two objects or subjects, in opposition to 二 which has a purely numeric meaning.
From the phonetic point of view, it also sounds better, as it is harder to misunderstand its meaning when in a sentence. Remember that in the north of China it is very common to add the sound 'er' (儿) at the end of words or sentences, such as in "你去哪?". But the composite sound "Liǎng" is much less frequent. It is easy to imagine lots of other misunderstandings that could occur in different words, such as "erzi" ("儿子"="one child", whilst "二子"="two children").
Learn Chinese Grammar 3 Time of a Day https://youtu.be/wJMoZj3as8g
For detail, you can check through youtube link
兩 & 二 1. 二斤 / 兩斤 2. 二尺 / 兩尺 3. 二圓 / 兩圓
