Well, I speculate he practice may in part come from praxis in large spaces where the tremulant/dissonance provided a more audible edge. Kinda like the reason why the Sistine choir may have developed its practice for the acoustical nightmare of St Peter's Basilica.

I would recommend highly accompanying chant with all the stops pushed in. This will do wonders for the chant. (And, if one wishes, he may use the tremulant to great effect with this registration!)

I use a flute 8' with a softer principal 4' for a cantor. When the entire choir and congregation are singing, I use a more substantial registration, as I would for hymns. I don't do much unaccompanied singing, because my choir isn't good at keeping pitch for any length of time. Also, I am not that fond of it, and a little goes a long way.

NOH?

8' soft flute or stopped diapason with a string celeste actually works rather well. Couple a 16' bourdon on the pedal and it has a nice warm bass feel.

I hope you won't mind if I answer several questions while answering yours.

1) If accompanying the Ordinary, I have two sets of stops ready, so that during the Gloria and Credo I can change at each double bar.

2) Reeds, celestes, tremulants, nazards, and others in similar veins should be avoided, because they draw more attention to the organ and the organist than to the text and the notes being sung. Usually, when singing hymns (such as Hyfrydol, Lauda Anima Mea, Lasst uns erfreuen) mixtures are appropriate, and reeds, too, but celestes and tremulants are more distracting than helpful in these situations.

3) Assuming that one intends to accompany the Propers ( inhale, everyone: I'm trying to take OP's premise) and assuming also that the congregation will not be singing these, play quietly enough that you support but do not overwhelm the schola. Furthermore, use the general principle that restraint is called for, not block-headedness. You're accompanying singers, attempting to be helpful, so proving that you studied composition, improvisation and modal music isn't your goal. Like concert black, your goal is to go un-noticed.

Exactly charles. That being said, I usually use flutes (all times) and quieter reeds or even principals (usually only if accompanying a congregation, if there's a lot of them).

The purpose of the organ in accompanying plainchant is normally to sustain pitch (in a schola) or keep a very large group of people singing together. Given that it has no other purpose and is effectively an intrusion, there should be enough of it to do what it's supposed to do - and no more! To this end, I think that an 8' flute is more than enough for a schola. For a congregation, things are a little trickier and it really depends on the scaling of the organ's pipework. I'd like to think that in a well built instrument, the 8' diapason on the main manual, and a single 16' on the pedal (with the diapason coupled through) should be enough. If the diapason isn't enough, I'd go for the 8' and 4' flutes on the main manual. If you have a Romantic organ with a very nicked diapason, maybe adding an 8' flute with better speech will round out the sound and provide enough. I cringe at the thought of adding the 4' principal, because then you're venturing into hymn registration territory - where the organ has the functions of leading and providing harmonic stuffing.

In short: less is more.

8' flutes

I tend to favor the flutes on the swell as they are normally less intrusive. Of course much of this depends on the instrument you have. Diapasons can be effective provided they are not overpowering. Sometimes flute 8 and 2 is what I use and it works well.

How does one avoid the melody notes when accompanying chant?

In other words bury the melody in the inner voices?

Who said a plenum for the ordinary (we are speaking of chant, aren't we)? 'Chant', when accompanied by a plenum is no longer chant. I have heard this done at Notre Dame in Paris and it is hideous.
For a congregational ordinary, if accompaniment is needed, it should not exceed eight and four principals, with a very questionable two foot thrown in only if it is absolutely necessary. More than this is just noise.

If we are speaking of a non chant ordinary the registration should vary according to the sections of text.

I confess I find some of the comments pseudo-derogatory and a bit baffling. I understand there are chant purists here. I love chant and I introduced it at my parish two years ago and there has been at least one sung chant at every mass since. During covid I’ve been exclusively using the Palmer-Burgess (N.O. Land so I have to sing in English most of the time although I still sing in Latin during communion). I love chant. Our parish also sings Latin ordinaries during Lent/Advent and we sing the seasonal Marian antiphons in Latin at the end of every mass. I also put special chants in front of the choir.

That said, we all know that most parishes are NOT adept at singing chant. Two years on with the same two settings people still tell me it feels awkward, and I review it every season and have laminated worship aids in modern notation to help them.

Organ accompaniment, simply put, is NECESSARY in many circumstances. And I understand that it is not ‘ideal’. But let’s be real: pure, monastic chant is not in the reach of the average parish. I use whatever accompaniment feels necessary given the circumstance. For the ordinary, I use a cornet because it’s the only thing that helps keep the congregation (and sometimes the cantors...) on cue. Soft flutes just wouldn’t cut it. Plain and simple.

I also don’t understand the disdain for Celestes in this context. Yes, the pitch is sharp on the celest, but it actually just complements the fundamental tones and broadens the sound slightly (assuming your strings aren’t sharp and overly keen). So I see absolutely no issue in using a string (or flute) and celest for the basic harmonies in the left hand with a solo sound in the right to play the melody.

I really don’t like the idea of falling into the trap of rigorous rules on “how it ‘must’ be done”. I’ve played in many, many parishes as a professional organist and know first-hand that what works in one parish doesn’t necessarily work in another.

Edit:
In addition to my comments above—
We know that not everyone sings in tune (or even close to it) so the congregation is its own form of “natural celest” anyway. Also, every organ is different and while “soft flutes” might sound great on one organ in one room, they might sound boomy, or anemic, or muddy on a different instrument in a different room.

As such, I think it’s dangerous to make edicts about how it “should” be done. Ironically, there are as many opinions as there are people on this forum, which is in itself an indication that there is great room for (reasonable and polite) disagreement. While the œvre of chant is relatively codified, its interpretation and accompaniment are not. (End of rant, lol)

ServiamScores

Don't let the people convince you they are as ignorant or dumb (musically) as they think they are or hope you will believe. Sadly, there are usually other 'reasons' that they will not mention, but simply say that it is not 'musically possible' to do what you ask. I think it is more a hoodwinked ploy of Hogwash.

I have lead children in a cappella chant for years. They have not yet developed the prejudice that it is beyond their humanity (human ability) to do so... adults, however, will not rise to the level of mediocrity and EFFORT unless challenged. We are a sad bunch.

under the assumption that if the people can learn to sing every verse of “On Eagle’s Wings” with their differing rhythms, then they can learn to sing a classic metered hymn they’ve never heard before. Same with chant. If you can sing some of the modern dribble, you can certainly sing simpler chants!

Bingo.

Point is, IMMHO the organ makes no difference really. I use it only for support and keep them all on pitch. If one is putting out the chant melody on a mixture or solo, they will become dependant on it in the wrong way.

Yesterday I played my recently composed accomp of the Salve Regina which I believe you have seen on the forum... (without melody and only an 8' prin). No one got lost, no one complained and they chanted up a storm.

@ServiamScores

A travesty at best... demonic, is a more apt description as you described in another thread. I salute you. Keep the Faith.

I would say use what works best for you using your organ, in your building acoustics, and for your singers, which you know better than anyone else. It's all relative and there is no standard that will work for everyone.

There are always exceptions. I have heard in my day, some seriously muddy flutes. I have also heard some grating principals. Use your own best judgement. It probably is more accurate than any opinions offered by outsiders unfamiliar with your unique situation.

Sorry for the typos. That should red (or 8' only) in the first paragraph and "playing" and "nor I are" in the 3rd paragraph.

I have to take issue with some here. Organ accompaniment for chant is necessary in certain situations. We have a volunteer choir composed of non-musicians from the faithful, but with nice voices. There is no way they can sing every Sunday all the propers from the Graduale Romanum without the organ which we had been doing until this Covid-19 circus came around. But with the organ they can with regular practice. This is much better in my opinion than singing the Rosinni propers or the texts to psalm tones which we would otherwise have to do.

For the choir I use 8' soft flute on the Swell. When alternating with the faithful in the pews for the ordinary, I will use 8' and 4' principal on the Great and a soft 16' on the pedals for them, to encourage them singing their parts: it makes for nice antiphonal chanting.

I think that for what settings/stops that are used is ultimately up to the organist's discretion to use. Most of the accompaniments that I see organist use for the Ordinaries is the Achille Brager's accompaniment to the Roman Kyriale. I use a slightly older set published by a French company, which does not employ so much full chords, but rather focuses on the main melody and simple harmonization with enough room for the organist's improvisation.
In my opinion what sounds are generally used are as particular as the organist playing the instrument.

A soft 16' Pedal with a discreet 8' added best sustains the voices, and thus maintains pitch.

Bingo. The pedal is what does the heavy-lifting as far as sustaining tone and pitch are concerned. Everything is grounded in the pedal. This means, in a certain sense, that you are afforded a certain flexibility in manual registrations if you're worried about keeping people on pitch.

Felicia

IMMHO a Soft 16 is good and avoid the Principal.

I like what Gamba said: "I don’t like to use 4s because then I’m doubling the melody an octave above the singers and it sticks out." I used a 4' last Sunday on the Missa de Angelis, and when I listened to it on livestream it really did stick-out. I'll try to stay with soft 8' stops. Thank you.