The foundational distinction in bitul b'rov runs between same-kind mixtures (min b'mino) and different-kind mixtures (min b'she'eino mino): the Rambam (Mishneh Torah, Forbidden Foods 15:1–4) rules that a forbidden substance mixed with a permitted substance of a different kind is governed by ta'am (detectable flavor), while a forbidden substance mixed with its own kind — where flavor cannot be detected — is nullified by a simple majority, so that even a piece of neveilah falling into two pieces of properly slaughtered meat is biblically permitted when the permitted pieces form the majority.
The Gemara's classic case of nine stores selling kosher meat and one selling neveilah illustrates a critical limit on majority rule: when a person purchases from a fixed, identified store and does not know which one, the meat is forbidden despite the majority being kosher, because anything fixed in place is treated as an even doubt — only when an item separates from the group and is found in the street do we follow the majority (Ketubot 15a:7, Chullin 95a:7, Pesachim 9b:10).
Three categories resist nullification even when a majority of the permitted substance is present: a leavening or flavoring agent sufficient to affect the mixture, and — most broadly — any item designated as a davar chashuv (an important, discrete entity); the Rambam (Mishneh Torah, Forbidden Foods 16:1–6) rules that any such forbidden item forbids the entire mixture in any quantity, and the Shulchan Arukh (Yoreh De'ah 110:1) enumerates seven specific items that qualify as davar chashuv, to which the Rema adds the principle that anything ordinarily sold by count (davar shebeminyan) likewise cannot be nullified.
A further categorical limit is that any forbidden item that has a potential avenue of future permissibility (davar she'yeish lo matirin) cannot be nullified even by a thousand permitted items, as the Gemara in Beitzah 3b:9 establishes; Beitzah 4b:3 then refines this by distinguishing rabbinic prohibitions, which may in certain circumstances be nullified by majority when no such matir exists, from biblical ones, which cannot be nullified intentionally at the outset.
For dry mixtures (yavesh b'yavesh) of the same kind, the Shulchan Arukh (Yoreh De'ah 109:1) rules that a non-davar-chashuv forbidden piece is nullified by a simple majority of two permitted pieces to one forbidden, and all three may then be eaten one at a time by the same person, while for different-kind dry mixtures the stricter standard of sixty to one is required.
The interplay between bitul b'rov and the mechanics of combining unlike substances appears in the context of wine libations in Avodah Zarah 73a:9, where Rabbi Yochanan holds that when a pitcher of water falls into a cistern that already contains yayin nesech, the forbidden wine is treated as nonexistent for purposes of calculating the permitted majority — but only if the water fell in first; if the forbidden wine had already fused with permitted wine of the same kind before the water arrived, it is roused and cannot be neutralized as easily.
The Gemara in Chullin 99:2 raises the question of why the gid hanasheh (sciatic nerve) cooked with other sinews is not simply nullified by the majority — a challenge that anchors the broader discussion of when a named, identifiable prohibition resists the ordinary rule of bitul b'rov — and Zevachim 73:1 states the general principle that any item subject to counting (davar sheyesh bo minyan) cannot be nullified even under rabbinic prohibition, and certainly not under a Torah prohibition.
When a spilled mixture cannot be measured afterward to verify that sixty-to-one was present, the Shulchan Arukh (Yoreh De'ah 98:2) rules that for a same-kind mixture a known prior majority suffices for permission, but for a different-kind mixture even a known prior majority does not permit it once measurement is impossible; and the Shulchan Arukh (Yoreh De'ah 111:3) rules that where two pots exist — one permitted, one forbidden — and a forbidden piece falls into one of them without knowing which, bitul b'rov at the biblical level may be relied upon to permit the mixture if the permitted pot's contents outweigh the fallen piece.
Igros Moshe (YD 3:12) offers a foundational analytical distinction: nullification of a minority within its own kind operates by negating the minority's very existence when it is imperceptible, whereas nullification within a different kind operates by the minority's substance becoming wholly absorbed — and he applies this framework to resolve apparent contradictions in the laws of orlah, firstborn animals, and tumah regarding when bitul b'rov takes effect.
Tzitz Eliezer (16:51) examines comprehensive principles governing majority rule in communal contexts, addressing the conditions under which a minority becomes bound by majority decisions, the enforceability of majority rule, and distinctions between different types of communal enactments.
Mishneh Halachos (3:103) addresses whether bitul b'rov applies to tumah at all, examining a difficulty raised against the Rambam's ruling that impure oil can be nullified within a majority of pure oil during Chanukah, and presents multiple views from Chazal and later authorities on whether tumah follows the same rules of bitul as other forbidden substances.
B'tzel HaChochma (4:9) addresses the status of a minority item that is unfit for a mitzvah once it has been nullified through bitul b'rov, concluding that the prevailing view among early and later authorities is that a nullified minority does not then combine with the majority to complete the required measure for fulfilling a mitzvah obligation.
Shevet HaLevi (3:104) examines disputes among authorities over whether bitul b'rov can resolve the status of a Korban Pesach when multiple animals become mixed, focusing on whether the act of cooking a defective animal can render it suitable and whether the resulting prohibition on eating is of biblical or merely rabbinic origin.