[a] Onigiri traditionally have sour or salty fillings such as umeboshi (pickled Chinese plum), salted salmon, katsuobushi (smoked and fermented bonito), kombu, tarako or mentaiko (pollock roe), or takanazuke (pickled Japanese giant red mustard greens).

Many Japanese convenience stores and supermarkets stock onigiri with various fillings and flavors.

On November 12, 1987,[2] lumps of carbonized grains of rice, thought to be riceballs, were excavated from a building belonging to the Yayoi period (2000 years ago) in the Sugitani Chanobatake Ruins in Ishikawa Prefecture.

The carbonized rice had traces which revealed that it was formed by human hands, thus it was initially documented as "the oldest onigiri.

"[2][3] In subsequent research, it was thought to be steamed and grilled, rather than boiled like today's rice, similar to another dish called chimaki.

[5] Before the use of chopsticks became widespread, in the Nara period, rice was often rolled into a small ball so that it could be easily picked up.

In the Heian period, rice was made into small rectangular shapes known as tonjiki so that they could be piled onto a plate and easily eaten.

[9][10] Other writings, dating back as far as the seventeenth century, state that many samurai stored rice balls wrapped in bamboo sheath as a quick lunchtime meal during war.

[citation needed] A packaging improvement allowed the nori to be stored separately from the rice.

When the packaging is pulled open at both ends, the nori and rice come into contact and are eaten together.

Miso is used as fillings, sometimes mixed with green onion, or spread over and roasted as a variant of yaki-onigiri.

Add condiments such as chives, miyakogusa, wasabi, grated ginger, nori, umeboshi plum, and pour hot Japanese-style soup stock.