This article is in process of being majorly rewritten.
There are four issues about what grains and/or pulses are allowed on Pesaḥ:
In its most literal sense, the five species mentioned in the Mišná that can be ḥaméṣ are believed to correspond to:
*) my own tongue-in-cheek term...
Later on (in post-talmudic times), a couple other grains have been claimed to belong to the five species — mainly because in the Western European setting of much of High Mediaeval Ashkenazi Jewry, spelt and emmer were not generally known, but rye and oats were.
Rye belongs to the same sub-family of grains as the genera of wheat (spelt (einkorn), emmer/durum, Timopheev’s Wheat, and bread wheat) and barley (2-, 4- and 6-rowed) — and as such, it is natural to include rye amongst the five species.
As for oat, it belongs to a different sub-family, and on a botanical basis, there is substantial doubt whether it should be included. However, centuries of minhág (religious custom) to include oat amongst the five species may be a weighty argument for being strict on this issue in those families that have that minhág. Those who do eat oats on Pesaḥ should be aware that, just like in the case of rice, there may be some degree of contamination from the five species. Therefore, one should only purchase oats before Pesaḥ, when it is halakhically acceptable to purchase foods that may have an accidental contamination of less than 1 in 60. Also, one should take care to purchase oats that are known to have a minimal degree of contamination. The lowest degree of contamination is generally found in oats from Ireland.
Also occasionally mentioned as belonging to the five species is millet (see, e.g., the Spanish and Portuguese Jews’ prayerbook publ. in London in 1965).
An early maḥlóket (rabbinical disagreement) is whether rice as a species can become ḥaméṣ. The established view is that it can not, and that as such, it is permitted on Pèsaḥ as long as it is not mixed with any quantity of the actual five species.
In Mediaeval Ashkenaz, a custom arose to avoid other foods that one thought could cause confusion about ḥaméṣ — thus actually creating confusion about ḥaméṣ. Kitniyyót may include most seeds, grains and pulses (legumes); as well as other foods that can be ground into meal of some sort — but which exact foods are considered to be kitniyyót depends on each sub-tradition, and even on each Rabbinical authority.
Some Sephardi traditions, especially in Europe, have also adopted this custom, although the “classical” Sephardi view is that kitniyyót are fully permitted.
Those Ashkenazi and Sephardi traditions which discommend eating kitniyyót during Pèsaḥ do NOT normally prohibit owning kitniyyót or using them for other purposes than human consumption.
Ashkenazim usually consider rice (see above) to be kitniyyót, whereas Sephardi authorities who discommend kitniyyót during Pèsaḥ may permit the use of rice.
Typically, kitniyyót includes pulses/legumes (peas, chickpeas (garbanzos), lentils, soy beans, peanuts, etc.) in dried form -- and, according to many, also in fresh form.
Non-ḥaméṣ grains, such as maize (corn), sorghum, millet (but see above) may also be included.
Some, but not all, also include non-grain and non-meal products of these — such as soy oil, peanut oil, etc.; and some also include seeds, like mustard seeds, sunflower seeds, etc.
[NOTE: Soy sauce is often fermented with wheat and is usually actually ḥaméṣ!]
Updated 20 September 2005 - 6. 17 Elúl 5765