Counting-rod unit digit eight

It marks eight in the units column of counting-rod notation.

Chinese mathematicians laid small rods out by place on a counting board to work in a positional decimal system. Counting-rod notation is attested by the 4th century BCE. The Nine Chapters, a Chinese mathematics manual dated around 100 BCE or 100 CE, describes procedures associated with rod calculation, though surviving evidence does not show every physical operation directly. Rod signs later appeared in written mathematical diagrams; abacus calculation later replaced rod work in China.

Why is the eight turned?

The place value determines the orientation.

Counting-rod numerals alternate orientations between neighboring place-value columns: 𝍧 is the units form and 𝍰 is the tens form. This helped distinguish adjacent places. A physical board could leave a zero place blank, while written notation could use 〇.

Sources from the Han dynasty assign the unit and tens orientations differently, so historical notation may turn the signs opposite to the forms encoded here. Fonts can also draw the rod strokes differently; a small shape change alone does not change the column assignment.