To put it simply, jinashi are mostly natural bamboo on the inside while jiari are mostly plaster or glue. Ji-nashi means “without” ji while ji-ari means “with” ji (地無し and 地在り). The word ji within the realm of shakuhachi can refer to any substance that’s applied and adheres to the inside of the bamboo bore, including bores which are cast using a mold or mandrel. Ji is used to augment the inner geography and effect playability. What’s potentially confusing is that jinashi can have a small amount of ji, even though jinashi means “without” ji. This will be explained in detail below. Additionally, both jiari and jinashi often have top coats of colored lacquer or urushi. To sum up, ji alters geography while urushi or lacquer just coats the geography. Think of painting an old piece of wood vs. coating the wood with plaster/ji, sanding it smooth, and then painting over it.
Historically, the first types of shakuhachi were of course jinashi and they had no ji whatsoever. They were simply called shakuhachi because jiari had not been invented yet, thus there was no need to differentiate. Around the end of the 1800’s, Araki Chikuo (Kodo II 1823-1908) was credited as the first shakuhachi maker to apply small amounts of ji to his shakuhachi. His students took this idea further by applying ji to the entire inner surface of their shakuhachi. Thus the jiari shakuhachi was born. The jiari quickly became the new standard, making jinashi largely relics of the past. However, jinashi are increasingly more popular, enjoying a full fledged renaissance in recent times.
As for crafting, jiari shakuhachi are easier to craft because a predetermined bore shape is cast or formed inside of the bamboo. The length can also be adjusted via joints. Conversely, jinashi are much harder to craft at a high level because the maker has to rely much more on what nature gives them. Because the bamboo plays a much larger role in the outcome of a jinashi, it means that acquiring suitable madaké bamboo is immensely important, and difficult. When crafted expertly, jinashi shakuhachi offer an entirely unequaled experience.
Additionally, there are subcategories of jinashi such as hochiku (法竹 aka hotchiku or hocchiku) and Kyotaku. Hochiku were created by Ishibashi Gudo at the request of master Watazumi (1910-1992). They tend to be much longer and larger on the inside than the jinashi that were used by the Komuso monks of the Edo period. Kyotaku are also longer bass shakuhachi which follow certain specific criteria such as having all the holes in a straight line and of a smaller size. These were invented by Nishimura Koku. Lastly, these longer bass shakuhachi inspired various “spiritual successors” such as Ken LeCosse’s large Taimu or “big nothing” shakuhachi.