Where is the line between opening principles and opening theory?


Well, first things first. You are playing daily games. Hopefully, you know that in daily chess people can look at opening explorer if they wish to do so.
To quote fair play policy:
- You may use Opening Explorer or other books without engine evaluations in Daily chess only (not in Online / Live play)
That being said, do you have to start learning opening theory? No, you don't have to.
Should you? Well, if you want to, why not, but not in a way to try to learn 20 moves by heart. You can play a game, and then afterwards, during the analysis you could look at the opening explorer to see if something went wrong in the opening, and if so, look what you could do instead. That way you can build slowly as needed without trying to be obsessive about openings.
You can of course try to learn what are the basic ideas of the opening you are playing and maybe to get to know some specific tactics.

I've only been learning the various names through the Explorer. I might have had a Gambit go wrong because my opponent read what I was doing, even though I was playing a variation. One thing I read was that you'd better know the theory behind the middle game after playing a certain opening.

I hate learning by rote and I don't depend on it. I do, however, seem to be able to remember sequences in Chess, just cause they're so cool. I think in patterns.

I've only been learning the various names through the Explorer. I might have had a Gambit go wrong because my opponent read what I was doing, even though I was playing a variation. One thing I read was that you'd better know the theory behind the middle game after playing a certain opening.
I mean, sure, it would be nice to know plans, structures and everything else in a certain opening, but that takes time and practice against people who know specific openings as well, and it will not happen on my level for instance, except rarely. I know because I do not have to memorize beyond move 6 or 7, except for some specific sharper lines I play.
And it will rarely decide games below certain level.
I can tell you this, I was over 1 700 rapid, by playing very long live games, games where you will have higher accuracy compared to 10 minute per side games. I don't really know theory too much, and yet, in most games against my peers I am not the one who knows less out of the 2 people playing. For some openings I know some characteristic moves, and some ideas behind those opening moves, and I rarely have a problem with the opening phase. You really don't have to memorize that much. It is more important to have some short plan in the middlegame based on the position and to not make big mistakes.
Mistakes will most often than not decide the outcome of games. Even though I get good positions out of the openings I play, I am still perfectly capable to lose any game.
My point is, sure, it will not hurt for you to watch a video or two about the opening and see if you have a playable position out of the opening and not constantly having to overcome +1.5 for the opponent.
But the less you know about a certain position, it will be more difficult to memorize moves as they will not make too much sense. That is why I've said to play and then build openings slowly. It doesn't consume too much time and you learn stuff in accordance with your games.
Just don't expect miracles. Openings are rarely what decides games on sub 2 000 level, probably even over that rating. The point is to have an ok position after 10 moves or so. Unless the opponent messes up, that is all you will get. It helps out a little but, but that is it.
All that being said, if you enjoy learning openings, all this advice is irrelevant.

So, the difference between opening principles and opening theory?
If you mean at what rating you should start learning opening theory, I've told you, you can start right away, but do not overdo it with too much memorization.
Basically, you can use the method I described since the beginning. So when you are 200 rated let's say, you learn what opening principles are, and you try implementing them.
After every game, you can take a peek at the explorer as part of your analysis.
People around 1 200 have some grasp over basic principles and they are implementing them, so of course you can start learning about openings you play, but as I've said in conservative way still.
If you mean when do you start some hard core memorization of moves up to move 20+, then I would say not for a long while, based on my opponents.

@nklristic Thank you. I think you nailed it on the head. I finally got some time to actually sit down and analyze my games, and it turns out I am weak in the middle, so, study tactics. I should also study my end game where I'm also weak. My openings are all in the 90 percentile, I even have a 100 against the strongest player I've played on here. I just got a copy of Kmoch which may be a bit premature, but so far is very interesting. I keep thinking that studying pawn structures, or how masters use their pawns, would be very beneficial to anyone. I'm back to the lessons. I would say the consensus on studying opening theory is that it can't hurt you, but it may not be beneficial until you get into the solid intermediate range. There are better things to spend your time on.

Theory = as the term is conventionally applied to chess openings, is the body of specific, documented variations comprising a specific opening. For example, the Najdorf Variation (a specific, commonly encountered and well-documented set of moves), is part of the "theory" of the Sicilian Defense.
Principles = just what the word says. Fundamental principles underlying opening play in general, which apply to all chess openings. For example - development, control of the center, King safety, etc. Thus "principles" are to distinguished from "theory", as defined above.
Chess Openings Resources for Beginners and Beyond…
https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell

Thanks, ya'll. I'm 65, played all my life and only got the bug about a year ago. The sheer volume of information easily available now is overwhelming compared to 30 years ago. Plus, I can ask stupid questions and have wonderful people like you answer them. Be grateful for what you have here, kids. You don't know how easy you have it.

Thanks, Trashtalker. If you're just going to be nice I might lose faith in you. Where's the trash talk? 😁 Come on hit me. ***** What I learned here is that it's not necessarily a theory, it's a system that you have to memorize. A theory let's me build something, and I can build many different styles of an object, say an airplane from knowing flight dynamics. Opening theory is specific instructions about how to build a specific object. And you better know how to carve the torso after you've carved the head or you are going to lose.

This is all just training for me so that I can go to an OTB tournament and at least not get my butt wooped by a bunch of kids. I thought I knew something about chess until I joined up here.

And I am really overwhelmed at what an incredible world community we have here. I can't say it enough. I am just astounded at the marriage of the Internet and chess.

The really great thing is that this is what the Internet was originally intended to do, exchange information. We leave out sex, politics and religion, something we've been told all of our lives is the polite thing to do in polite society, and for the most part everyone pretty much behaves themselves. Except a few trash talkers.😁 If they would just leave all that out of social media maybe there wouldn't be so much misinformation floating around. Then it wouldn't be as interesting.

Opening theory is memorizing moves, opening principles is understanding why those moves are being played.
Nothing wrong with studying openings, but you should not blindly memorize them. Instead you should learn why the pieces are being moved in a certain way and understand the ideas behind those moves, optimal square that your pieces should go, most common tactics that can arise from those positions, how to punish bad moves from your opponent, tactical motifs to keep an eye on, etc etc...

Improving Your Chess - Resources for Beginners and Beyond.....
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell

"Studying openings" will mean different things to different people.
What exactly do you mean by "study"?
If you mean finding an opening you enjoy playing and wanting to learn more about it? Sure, do that. Play over Master games. Start to gain some understanding of "why" the pieces and pawn go where they go.
If by "study" you mean memorizing moves with no basic understand "why" those moves are played? You're doing it wrong.