R Yonasan Sacks (now RY of Lander College, then of YU; then and now of
the Agudah of Passaic) once gave a shiur which gave a taxonomy of kinds
of derabbanan.
I took notes, and blogged it shortly after at
<https://aspaqlaria.aishdas.org/2006/02/07/types-of-halachic-rulings>\.
Since we're going back around 20 years, I will just quote my notes (which
is still a few years later an embellished with things I learned when
sharing with Avodah.)
RZL and RMR were discussing the definitions of minhag, taqamah, gezeira
and siyag. Here's my notes:
The following taxonomy of kinds of halachic ruling was culled from the
Rambam, Hilkhos Mamrim ch. 2, and includes thoughts learned at a shi'ur
given by R' Yonasan Sachs...
1. Minhag. Custom. Custom, although not really part of halakhah, can
change. Minhag is any act that the masses, on their own, accept.
According to the Rambam, to qualify as a minhag the practice must
then ratified by the rabbinate. Any minhag that is against actual
halakhah, is called a minhag ta'os, a mistaken minhag. Any that is
based on a misunderstanding is a minhag shetus, a foolish custom.
These two subtypes should not be followed. Any nearly universal
minhag is called a minhag Yisrael, and has most of the stringencies
of law. Yarmulka and ma'ariv services are two examples of a minhag
Yisrael.
Notice that it's anything the masses accept to do. Which would include,
I would think, Yom Tov sheini shel goliyos when qiddush was al pi re'iyah
and they had pragmatic necessity to keep both days.
But also, we get into a homonymity. There is capital-M minhag, which is
a category of mandatory practice, and then there is lower-m minhag, which
has lots of meanings. Like when a qehillah accepts the pesaq of Rav X,
we may say there minhag is to do Y, but really it's halakhah, not Minhag.
Or a hanhagah tovah that wasn't ratified by the rabbinate, we would still
say "minhag avoseihem" even though it's not a Minhag in that sense, yet.
And I think that's what Abayei wrote the people of Bavel about YT Sheini.
Your ancestors for practical reasons had a lower-case-m minhag, and I am
now turning it into a capital-M, binding, Minhag, applicable even when
the reason doesn't.
2. Din deRabanan. A rabbinic law. These are set up by the rabbinate,
instead of the masses, in order to preserve the spirit of the law.
For example, Purim and Chanukah. There are 7 new commandments that
are entirely rabbinic. According to the Rambam, who only counts
biblical mitzvos amongst the 613, this means there are actually 620
mitzvos altogether.
3. Gezeira deRabanan. A rabbinic "fence". These are enacted to prevent
a common cause for breaking the act of the law. For example, one
may not place food directly on a fire before Shabbos in order to
keep it heated during Shabbos. This is a fence around the law
against cooking on Shabbos. To prevent the gezeira from being
violated, a metal cover, called a "blech" in Yiddish, is placed on
the stove top before Shabbos with the flame (turned to a low
setting) under one section and the pot with food placed on the
blech. This blech serves as a fence, allowing heating of the food
without any danger of violating the law. Note that a "gezeira
dirabanan" becomes binding only if the community accepts
it.According to the Rambam, a gezeira cannot be overturned.
However, a gezeirah where the law's purpose is included in the
legislation is implicitly conditional on the purpose. The problem
is in knowing when the purpose is given in the quoted gezeirah, and
when the gemara provides a motivation on its own, after quoting the
gezeirah. For example, meat must be salted within three days of
slaughter, or the prohibited blood will be too soaked into the meat
to be retrieved. What about the contemporary situation, where meat
is generally frozen solid? Some rule that since the reason is given
in the legislation, and the reason doesn't apply, neither does the
time limit. Others rule stringently, presumably because they do not
believe the reasoning about the blood being soaked into the meat
was part of the legislation as initially codified.
According to the Tif'eres Yisrael (Ediyos 1), there are actually
two sub-categories:
1. Siyag. Fence (Hebrew; "gezeirah" is Aramaic). Something that
will lead to a future violation to do an error in
understanding the law. Such as the ban on mixing poultry and
milk, lest people become lenient in mixing meat and milk.
2. Cheshash. Concern. Cases where the threat of violation is in
the current situation, because one is in a circumstance where
habit taking over or other accident is likely.
The Tif'eres Yisrael says that a cheshash can be deemed
inapplicable if the norms change such that the threat no longer
exists. It does not require a beis din that is greater in number or
wisdom as the law is not lifted, just that the current situation is
deemed to be outside the limits the law addressed.
4. Asmachta. Mnemonic. The Raavad (on Mamrim 2) considers laws backed
by a mnemonic in the Torah are in a different category than other
rabbinic laws....
The distinction between the second and third categories is subtle. In
order to be a din (or issur, or melakhah) deRabanan, the prohibited
action is one that is similar in purpose to the permitted one.
In contrast, a gezeira does not even require an action. In the example
I gave, it was inaction, leaving the pot where it is, that is
prohibited. Second, the category includes things that are similar in
means to the prohibited act, and will therefore cause confusion about
what is and what isn't okay; and things which will allow people to be
caught up in habit, and forget about the prohibition. Only a gezeira
may defy an actual Divine law (although a pesaq will often define one),
and even so only under specific circumstances. All of the following
must be satisfied:
* The law being protected is more stringent than the one being
violated. This determination isn't easy.
* The law is being violated only through inaction. No one is being
told to actively violate G-d's commandment.
* According to the Ta"z, the law being violated will still be
applicable in most situations. It still must exist in some form.
(Not every acharon agrees with this requirement.)
In another way, a gezeira is less powerful than a normal rabbinic law
in that it cannot be compounded. One may not make a "fence" for the
express purpose of protecting another "fence".
A law is considered accepted if it becomes common practice. Any din or
gezeira that does not get accepted by the masses in the short run, does
not become binding in the long run. Similarly, there are rules for
pesaq, but they are violated if the masses choose to follows some other
rabbinic body's pesaq. Notice, however, that this need for acceptance
is only in the short run, to enact the law. Once a law is accepted, it
may only be overruled by pesaq. It does not cease to exist just because
it faded out of practice.