じしゃく の りょうはし は、Nきょく と Sきょく と いいます。
同じ きょく は しりぞけあい、反対 の きょく は 引きあいます。
じしゃく を いと で つるす と、北 と 南 を さします。
じつ は、地球 ぜんたい が 大きな じしゃく なのです。
Why six school grades, and not N5–N1?
Because that is how Japanese children learn to read. Japan's Ministry of Education fixes exactly which character belongs to which grade — so "grade 3" is a boundary you can verify, not a feeling. Every article here is machine-checked: if a single character sits above its grade, it is not published.
The result: you never hit a character you have not learned. That is the whole reason this product exists.