The big question on everyone's mind is, How many characters are there, and How many do I have to learn? These are reasonable questions, though the answers are not as straightforward as the questions. The biggest collection I am aware of had over 80,000 characters, but that number may be misleading because a certain number of those characters are variants of other characters, i.e. they are the same character, just written slightly differently. If we take away the variants we can say there are around - and this is a very rough number - 50-60,000 characters. Today, however, most of those are obscure, meaning they are no longer in use. Here is the characer with the most strokes - 64. It is four dragons, and meant verbose or long-winded. (The character with the least number of strokes is 一 yī meaning 'one.')

So how many characters are in use today? The big dictionaires have 8,500 to 12,000 characters; when a Chinese person finishes high school they probably know around 4,000; a very well-educated Chinese person probably knows upwards of 6,000. 3,000 will get you very far along in terms of the percentage of the language you will be able to understand, over 98%. This is a bit misleading because most Chinese words are made up of two characters, which means that more important than the number of characters you know is the number of combinations, ie, words. You could know the meaning of two characters and not be able to figure out the meaning of a word made up of them. One example is 马上. 马 means 'horse' and 上 means 'on,' so what do you think 马上 means? (The answer is at the bottom of the page.)

Chinese characters have been around for approximately a gagillion years - wait, that can't be right - how about 3,500 years. The earliest records we have are characters carved into tortoise shells that were used for divination - telling the future - like who would win tomorrow's battle. We can follow the evolution of the characters by looking at later records. The following image, taken from https://www.omniglot.com/chinese/evolution.htm, gives a good overview of the process.

Each of the following videos shows the evolution of a common character from its earliest appearance until now. We know this because of examples of the characters that have been found on artifacts from the various periods.

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Brown University intro

Answer: 马上 means immediately (because back then, the fastest way to get to a place was on a horse.)