Minnek I (technical)
Different things are called minnek1, usually translated as 'custom'. It can denote one out of several concrete ways of doing something, in particular something that is seen from outside as ritual. An example: Litvakes, Polakes, German Jews and Alsatian Jews all say sliches in the days around Rosheshone, but they don't have (necessarily) the same sliches for a given day - in fact, this often used to be different from village to village.
It can also mean doing something in addition. Example: Chasidists have the minnek of "opsheiren", cutting a boy's hair for the first time on his third birthday. My Gateshead cousin once mixed these meanings when he remarked "Yekkes have a lot of minhogim, just like the Chasidim." What he probably meant was Yekkes wash before kiddesh, and most others make kiddesh first. (But all wash, make kiddesh over wine, and eat bread - it's just about the concrete execution.) Chasidists however feature all kinds of funny novelties at a third kiddesh in the afternoon.
In addition, minnek in Chôshen mishpet has a different aspect and meaning. And, for the sake of completeness, minnek can also mean the whole set of menogem, similar to nusech.
Anyway, what I really want to suggest is a classification of legitimacy. Please feel free to discuss this, it's more a draft, and the numbers and letters are just for reference, not to imply an exhaustive analysis:
1. On equal foot
There are two or more menogem, none of which can be shown to have more legitimacy in terms of din, history or other. You follow your tradition.
Example: Choice of sliches.
2. Different reasoning, and only one makes sense
The reasons for the conflicting menogem concerning a mitzve are known. This is much more difficult, because a situation might arise where you come to the conclusion that only minnek A makes sense, but your tradition is to follow minnek B. In fact, if this is so clear, and no proponent of (your own) minnek B can convince you that it makes at least equal sense, you should abandon your minnek.
Example: Calculating the times of the day according to a day that lasts from dusk till dawn ("shittes Mogen Avrohm"), or from sunrise till sunset ("shittes haggônem", a. k. a. "shittes haGro").
3. Different reasoning, but they all make sense
This is easier, because that makes them actually equivalent, and you follow your tradition. Of course, in real life, this and #2 aren't so clearcut, and you might have a case where both rationales do make sense, but one is stronger.
Example: Washing before or after kiddesh.
4. Other conflicts
There is a saying that claims minnek invalidates the (Jewish) law. That is not so, except for certain cases in financial law, where minnek is a different concept, as I mentioned above. So, where there's a conflict between a law and a minnek, for example one of the chumre type, the custom has to cede.
Example: Say, in a community it's common to consume milk only when it's been supervised by an Orthodox Jew right from the milking on. The rabbi's stance is that FDA supervised milk is actually kosher, but that it's the minnek to have the other one. If somebody is offered an "FDA milk" dish by somebody kosher and declines it, he might violate the law that forbids to abash people.
5. Making use of a difference
If you can't do something because it's the minnek not do, even if the strict letter of the law allows it, a situation might occur where somebody who has a diferent minnek can do it (even specifically for you), and you benefit from that. That might seem like a foul trick, but it makes sense if you keep in mind that the law does allow this in the first place. It just starts to cross the line if it is done with differences in minnek that are based on differences in the interpretation of the law, because that would actually imply you think it's forbidden, but you don't mind if another one breaks the law for your benefit.
Examples: "Sfardiya shel shabat", who handles certain food for an Ashkenazzi on a shabbes blech in a way that is understood to be forbidden by Ashkenazic authorities.
Now for types:
A. Traceable menogem
Earlier, there was a uniform minnek, but at a certain point, a minnek was changed or introduced. Here it depends heavily on the circumstances, as the next categories will show, but certain menogem were introduced as takkones and so are in fact binding for the community in question, though not necessarily for others. It is a question if this is still binding if you change the community, if the community is discontinued, and if the cause for the change is no more extant. In general, historic cases show that the minneg lives on, but this might not be true for this category. (Several years after the Jews of Worms were dispelled, others went there, and they strictly adhered to the minnek of Worms.)
Example: During WWI, the Würzburger rav ruled to say "shômer yisroel" in Tachnun every workday in Würzburg, instead of only on fast days, as per the Ashkenazic minnek including the older Polish minneg.
Chumres can be personal, but there are also menogem that are basically chumres. Though the scope for a rabbi to disregard this kind of rules under given circumstances is obviously wider than with actual mitzves, the principle is that they're binding for the pertinent group.
Example: waiting three (Germany), four (Iran?), more than five (Eastern Europe) or six (Morocco) hours before eating dairy food after having eaten meat, while the law demands only one hour.
(Note: I don't touch on similar notions like gezeires here, because their implications are different.)
C. Nonsensical menogem
If they are otherwise harmless and it's your tradition, go ahead. But they usually aren't, for instance because of illegal imitations of non-Jewish practice or maybe also because those who introduced them weren't entitled to at the time.
Example: Kappores hinkel, in the eyes of the Sefardic Shulchen Orech a minnek shtus.
D. Erroneous menogem
Similar reserves like concerning nonsensical menogem. Many of these are rather recent and came about as changes to what thousands of years of chachomem didn't find problematical, or they were taken over from other contexts where they might have made sense.
Example: Putting off tefillin on roshchôdesh and chollemôed before mussef. The origin (keser vs. keser) might be regarded a nonsensical minneg anyway, but in addition, this is erroneous (minneg toes) in nusech Ashkenez, because there, you don't even have a kedushes keser at all, and so, no "conflict" of keser vs. keser.
E. Menogem of dubious or illegitimate origin
Well, mainly Shabse-Tzvi related, but depending on your cognition and to a certain degree outlook, this might include chasidist and Liberal innovations or the earlier novelties of the seifer hazzôer. Those menogem should probably be abolished even if they're harmless "as such", because their continuation gives legitimacy to their sources. An aspect that shouldn't be overlooked, though, is that whoever is the first to be associated with a minneg in a surviving source might well have adopted it from a contemporary community or the like.
Example: Saying Ledovid h' ouri veyish-i in Ellel might well be an example.
F. Obsolete menogem
Those might be a subdivision of erroneous or nonsensical menogem. On the one hand, menogem are worth to be kept, on the other the question often is if the case is one of minnek at all. If not, then there's at least no duty to continue a habit.
Example: Eastern European winter clothing at 40°C (104°F) and high humidity.
1. Ashkenazzi pronunciation ['mınək], plural [mə'no:gəm] or [mın'ho:gəm], Christian pronunciation with Ukrainer accent [mi'na(·)g], sometimes [min'a(·)g] or [min'ha(·)g], plural [mina'gi(·)m]. I'm writing the singular with a -k because in English, a -g would actually be pronounced as a voiced sound. (In case you see a box instead of a schwa sign, you might consider changing your browser.) (back to text)