He then wrote the rod numerals according to their place value:  In Japan, mathematicians put counting rods on a counting board, a sheet of cloth with grids, and used only vertical forms relying on the grids. An 18th-century Japanese mathematics book has a checker counting board diagram, with the order of magnitude symbols "千百十一分厘毛“(thousand, hundred, ten, unit, tenth, hundredth, thousandth). Examples: Rod numerals are a positional numeral system made from shapes of counting rods. Positive numbers are written as they are and the negative numbers are written with a slant bar at the last digit. The vertical bar in the horizontal forms 6–9 are drawn shorter to have the same character height. A circle (〇) is used for 0. Many historians think it was imported from Indian numerals by Gautama Siddha in 718, but some think it was created from the Chinese text space filler "□", and others think that the Indians acquired it from China, because it resembles a Confucian philosophical symbol for "nothing". In the 13th century, Southern Song mathematicians changed digits for 4, 5, and 9 to reduce strokes. The new horizontal forms eventually transformed into Suzhou numerals. Japanese continued to use the traditional forms. Examples: In Japan, Seki Takakazu developed the rod numerals into symbolic notation for algebra and drastically improved Japanese mathematics. After his period, the positional numeral system using Chinese numeral characters was developed, and the rod numerals were used only for the plus and minus signs. A fraction was expressed with rod numerals as two rod numerals one on top of another (without any other symbol, like the modern horizontal bar). The method for using counting rods for mathematical calculation was called "rod calculation" or rod calculus (筹算). Rod calculus can be used for a wide range of calculations, including finding the value of , finding square roots, cube roots, or higher order roots, and solving a system of linear equations. Before the introduction of written zero, there was no way to distinguish 10007 and 107 in written forms except by inserting a bigger space between 1 and 7, and so rod numerals were used only for doing calculations with counting rods. Once written zero came into play, the rod numerals had become independent, and their use indeed outlives the counting rods, after its replacement by abacus. One variation of horizontal rod numerals, the Suzhou numerals is still in use for book-keeping and in herbal medicine prescription in Chinatowns in some parts of the world.