Matza can be made from any one of the five grains: wheat, barley, spelt, oats, and rye. It is a hiddur mitzva to use wheat for the matzas mitzva. (ShA1 — MB1,2)
The Shulchan Aruch allows cooking rice and other legumes on Pesach. The Rama says that minhag Ashkenaz is to be machmir. Do not eat kitnios especially rice and millet. (ShA1 — MB7)
Cases where kitnios are allowed: (a) some fell into Pesachik food and disappeared; (b) nothing else to eat; (c) for benefit — e.g. kitnios oil for fuel; (d) have them in the house. (Rama1 — MB8,10)
Sprouting grains are chometz gamur. (MB20)
Wheat used for matzos at the Pesach sedarim must be guarded from becoming chometz for the sake of the mitzva. Make sure no water contacts the wheat once it is harvested. The minhag is to harvest wheat while it is still green. (ShA4 — MB22)
If you cannot obtain wheat guarded from the time of harvest, make sure it is guarded from chometz at least from the time it is ground into flour. (ShA4)
Regular flour bought in the market has been pre-washed. Consider it chometz — do not use it even if there is no matza available. Many poskim hold you are not allowed to have such flour in the house on Pesach. (MB24)
There is a holy custom to eat only matza shmura the whole Pesach long — guarded at least from the time of kneading, if not from the grinding. (MB25)
