In the same way that matter is made up of atoms that combine in different ways to create our physical world, radicals are the building blocks of Japanese and Chinese characters. Radicals are ideographic subunits that carry clues about the meaning or reading of a character in the Japanese and Chinese languages; for example, one radical meaning water is often found in characters with meanings related to water, and similarly, other radicals perform the same function with meanings around earth, fire, motion and movement, parts of the human body and many others. If you can recognize the radical in a character, you can often guess its meaning or pronunciation even if you have never seen the character itself before. Below are some examples of kanji with shared radicals highlighted in blue (water) and red (fire).

Learning radicals is critical to studying Japanese and Chinese, but it is not always easy to grasp the full universe of radicals and how they relate to all the characters in either language. In Japanese, there are more than 2000 characters (kanji) that are specified by the Japanese government to be essential for "daily use"; known as the jouyou kanji (常用漢字), these characters are taught in primary and secondary school to all Japanese students. For non-native speakers of Japanese, learning these kanji is a lifelong process, one in which the vastness of their readings and meanings continually unfolds in new contexts.

This visualization seeks to illustrate this unfolding in a literally physical sense — one in which radicals are likened to atoms, from which molecular kanji arise. There are around 250 unique radicals that are found across all kanji (within the jouyou kanji and beyond), nearly all of which make appearances in kanji for daily use in many different possible combinations. Japanese students encounter these radicals in primary (grades 1 through 6) and secondary school education following standardized curricula, while non-native students of the language are exposed to them in less standardized ways. In any case, it is not always easy for students in either situation to cultivate a bird's eye view of the kanji they are learning and how they relate to others.

In this visualization, the universe of Japanese kanji comes into view as you step through the school years of the typical Japanese student. In each single full iteration, all of the kanji introduced in the respective school year materialize, along with their composite radicals. Each red circle represents a radical introduced in kanji for a given year, each of which is linked with small black circles representing those kanji. The gold colored circles represent radical clusters — radicals that appear in multiple kanji over each year. Gray lines indicate connections between individual years' radicals or kanji; red lines indicate links between radical clusters by way of shared kanji between them. As each radical cluster increases in radius (size), it becomes possible to see the frequency with which those radicals appear in shared kanji across the Japanese language, growing year over year as students study kanji in primary and secondary school.

For readability, this visualization only shows 20 of the most common radicals and their kanji, and only those kanji which are taught in Japanese grades 1 through 6. In each of the sections below, a list of kanji learned by students is provided according to grade level.

This visualization was built in D3.js by Steven Braun. The data are taken from Jim Breen's KRADFILE, a concordance of all kanji and their composite radicals. The code for this project is freely available on GitHub.