If you have studied Chinese for a long time, one thing you might have noticed and felt strange about the language is that we have two sets of characters for numerals. In most of the languages, this is not gonna make any sense simply because — why on earth do you need other characters for numerals which can already be written with such simple strokes? Actually, I myself, like many other native Chinese, also got puzzled by the fact there's another set of way more complex characters for numerals in my childhood time. But after I found out the reason later, this fact suddenly became rather rational and necessary to me. For those who are also confused by it, I'm going to reveal the real reason of having two sets of numeral characters.

Firstly, I'll briefly list those two sets of characters with the corresponding Arabic Numerals here.

零——零（〇）——0

一——壹——1

二——贰——2

三——叁——3

四——肆——4

五——伍——5

六——陆——6

七——柒——7

八——捌——8

九——玖——9

十——拾——10

廿(niàn)——念——20

百——佰——100

千——仟——1000

万——万——10000

亿——亿——100000000

The more complex set of characters is called 大写数字(Capital Numerals) and the simpler one, 小写数字(Lower-case Numerals). The only difference between them is that Capital Numerals are used specifically in writing the amount of a cheque, a remittance note or any formal bank statement together with the Arabic Numerals. The reason in doing so is to make it much harder, in fact almost impossible, to change the number in those statement by altering strokes. Since both Lower-case Numerals and Arabic Numerals are structurally simple and easy to be changed by adding a few strokes, these complex characters are introduced to prevent cheating and fraud. You now may be thinking that people had invented those characters, but they've been existing for thousands of years already only with different meanings in ancient times. Here're the original meanings of these characters:

壹——专一的 single-minded; totally concentrated and focused

贰——变节，叛变 to betray; to start a rebellion

叁——“参”的另一种写法，加入，接见 an alternative way to write 参(cān) meaning "to join; to receive somebody"

肆——任意妄为 to be rebellious and do whatever one is desiring irresponsibly without discernment

伍——军队中的单位，五人为一伍 a unit in military that consists of five soldiers is called a 伍(wǔ)

陆——高出水面而平坦的陆地 a flat land above the water

柒——漆树，漆料 lacquer; trees from which lacquer is made

捌——一种用于聚拢谷物的工具 a tool used for gathering and piling grains

玖——黑色的美石 a black and exquisite stone

拾——捡 to pick up

念——惦记 to think of something continuously

佰——统率一百人的军官 a military officer who is the leader of a troop of a hundred soldiers

仟——统率一千人的军官 a military officer who is the leader of a troop of a thousand soldiers

Then here comes another interesting question for us to think about: it seems that most of these characters are completely irrelevant to numbers, so how was the idea of using them to record money invented? Here's the story:

The idea of using these characters for numbers was first raised up approximately 1300 years ago during the Tang Dynasty (唐朝) when Emperor Wu Zetian (武则天) reigned the country. 武则天(wǔ zé tiān) was one of the most famous, powerful and wisest emperors in China's history and also the only female emperor ever (notice I deliberately avoid using "empress"). She was one of the most creative and innovative people in ancient times who dared to create new characters for her own. Among those characters created by her, two of those preserved became the most well-known. One is 曌(zhào), referring to the moon shining brightly on the sky, and it was used as Wu Zetian's official name to show off her majesty and wisdom. The other one is exactly the alternative way to write the capital zero, 〇(líng). Something amazing and fascinating about this character is that it's the only one that ever existed in China's history written with a non-standard circular stroke. And for some reasons that still remain unknown, she first came up with the idea of replacing numerals with Capital Numerals. 

Although Wu Zetian was regarded a rather controversial historical figure even during the time in Tang Dynasty, her idea about Capital Numerals was well preserved somehow. By the time Song Dynasty (宋朝) took the place to rule the country, all arithmetical numbers in official document of the government had to be written with Capital Numerals. And the government adopted this measure intentionally for the purpose of preventing embezzlement. A scholar of Song Dynasty, Cheng Dachang (程大昌), wrote in his book 《演繁露·十数改用画字》that 今官府文书凡计其数，皆取声同而画多者改用之。于是壹、贰、叁、肆之类，本皆非数，直是取同声之字，借以为用，贵点画多不可改换为奸耳。(Currently, all official document and statements have adopted homonyms with complex strokes to replace simple numeral characters as long as they are to record numbers for arithmetical or accounting purposes. Here the characters used, such as 壹, 贰, 叁, 肆, were all not referring to numerals originally. The government just selected homonyms for numeral characters and adopted them into use, valuing the fact that they cannot be tampered with for evildoing due to the complexity of the strokes.) This shows that the Song government had already realised and valued the advantages of using Capital Numerals as in prevention of cheating in economic activities.

However, the use of Capital Numerals wasn't written into the law by strict legislation and widespread until the Ming Dynasty about 700 years ago. During the first emperor of the dynasty, Zhu Yuanzhang (朱元璋)'s reign, a huge and extremely serious corruption case was reported, which had astonished the whole country afterwards. Guo Huan (郭桓), the contemporary 户部侍郎 (Vice Minister of Finance), conspired with a number of officials from 刑部 (Ministry of Police Force), 工部 (Ministry of Land and Construction), 兵部 (Ministry of National Defence), 礼部 (Ministry of Education and Diplomacy) and governors and lairds from many provinces to embezzle grains, salts and other agricultural products which were supposed to be collected as taxation during that time. The amount of the embezzlement, converted to rice grains (which were the standard way to calculate tax), was about 120 million kilograms, which was almost equal to the amount of grains actually received by the government as taxation for a whole autumn. The emperor was extremely infuriated by the corruption case and executed thousands of officials involved, with a countless number of officials imprisoned, exiled and convicted. Ever since then, it was strictly legislated that all characters used for arithmetical recordings in accounting documents must be changed to Capital Numerals, which has been preserved till today.