Is the QGD "Classical" variation Bf4 or Bg5 ?

ECO calls D37 the Classical var. , giving typical line 1 d4 d5 2 c4 e6 3 Nc3 Nf6 4 Nf3 Be7 5 Bf4 . Yet a chess.com lesson titled QGD classical variation, by GM Kaidanov, shows 4 Bg5 . Running his stem through 4 Bg5 in 365chess Opening Explorer yields D50, and could lead to a variety of lines D50-D69 including Lasker, Tartakower & Orthodox but not "Classical". 

van der Sterren in FCO does not use term "Classical". He calls above, through Bg5, the Capablanca line. Ntirlis in Playing 1 d4 d5, A Classical Repertoire simply lumps lines into QGD Bg5, QGD Bf4 and Exchange. He gives no other names for these lines and never uses ECO codes.

chesspub does not use "classical". It just discusses 5 Bf4 in same section with Tarrasch, semi-Tarrasch & Exchange var.

Studying openings has been compared to having dental surgery. Hard enough without agreement on names. When I use a word it means just what I want it to mean... Humpty Dumpty.

If anything, I'd interpret the "Classical" moniker to mean the defenses that play e6. as in the Lasker variation, The Tartakower Variation or my favorite the "Orthodox Variation" The Orthodox is solid as a rock, never been refuted and is the where many of the "newer" Queen's Gambit Defense got their origins. At one time, the Orthodox was the primary defense. I continue to play it. Like the Petroff... the Orthodox Var is a defense with such defensive capabilities, White has to play extremely exact, to get, and to keep any advantage... hence resulting in the high % of draws in it's heyday... Now the theory (which is as complex as it gets) is unknown to the masses (Bonus!)

The Orthodox hype aside... Both Bg5 and Bf4 are part of the Queens Gambit (so is playing e3 and and temporarily shutting in the DSB)

Bf4 var is known in the United States as the Horowitz Attack, named after ISRAEL ALBERT HOROWITZ, he was one of (if not the best) American players in the 1930s. He authored a lot of great chess books. It's also known as the Blackburne Attack in Europe, Named after Joseph Blackburne, a famous English Chess player Pre WWI

The Bf4 var as far as tactics goes is IS RIGHT UP THERE ON PAR with the Sacilian Nadjorf, the Semi Slav Botvinik var.. in fact I've read some expert think it might surpass them as far as complexity goes. Whatever the case, you are entering into a pure tactical matrix if you play the Horowitz Attack!

Variations where White plays Bg5 are positional in nature, both sides tend to fight over small advantages, i.e. who can stiff who with an extra move in the opening (i.e. the fight for tempo) Some variations like the Tartakower have in one of it's main lines hanging pawns at d5 & c5... Yet..... in variation where e6 is played first (excluding the Slav and Semi Slav varaitions) about 50% of the time the resulting pawn position will be the famed "Carlsbad" pawn formation where White will launch a minority attack and Black to off set this, will launch an attack on White's king.

Good stuff all

Thanks kindly for your reply. Suspect I rely too much on the old ECO chart. My study of openings consists of little more than searching the ECO listings to see what game I played. It does use 'classical' for D37, but that word, like 'natural' could mean anything you want.

Read somewhere that Bf4 was a favorite of Botvinnik and also have seen it called the Harrwitz attack.

Maybe it is called the Harrwitz Attack... I always thought it was Harowitz .. and named after I.A Horowitz ... Don't quote me on that ... For years I've thought that,,, I'm going to check into that..

5. Bf4 is a "classical" move because Steinitz played it quite a bit in his last years and Zukertort also tried 4. Bf4 in the first World Championship match against Steinitz (but they quickly reached an IQP position).

Addendum. QGD Classical seems to be another name for Orthodox.

Found a book that uses term 'Classical'. James Rizzitano's The Queen's Gambit Declined , 2007. Last chapter is 8 Classical Defence . Line given is 1 d4 d5 2 c4 e6 3 Nc3 Nf6 4 Bg5 Be7 5 e3 O-O 6 Nf3 Nbd7. This line, through 4 Bg5, is given ECO code D50, range D50-D69 including Lasker, Tartakower, Orthodox and a few unnamed. Rizzitano does not have a chapter on Orthodox and nowhere uses that term.

Wikipedia article on QGD describes Classical as 1 d4 d5 2 c4 e6 3 Nc3 Nf6 4 Bg5 Be7 5 Nf3 O-O 6 e3 Nbd7 . Reference is Hooper & Whyld Classical Variation 1996. But this would be coded as D60 Orthodox.

chesspub probably has most recent comments on lines and names. That site divides QGD into two "Sub-Boards". First is QGD Mainline : Lasker, Tartakower, Ragozin, Vienna & Cambridge Springs. Second is titled QGD Intro. This includes Tarrasch, semi-Tarrasch, Exchange & 5 Bf4 . There are a great many posts & I could only skim a few dozen. Classical is not mentioned anywhere. Three posts discuss Orthodox. With some transpositions, the lines correspond to the line in the chess.com lesson which GM Kaidanov called QGD classical variation .

Yes, it's a bit confusing. But like you pointed out, the "Orthodox" is mentioned far more often than the "Classical".

My understanding is that Bg5 is the mainline, and after Be7 we move further down the line until the defining moves branch us into the main three: Lasker (...Ne4), Tartakower (...b6), and the Orthodox (...Nbd7).

I've even seen some refer to the "Classical Variation" as a specific line within the Orthodox (where White delays Bd3 in favor of Rc1 first, while Black plays ...c6.) For example: "Queen's Gambit Declined, Orthodox Defense, Classical Variation".

But at that point I believe we're just muddying the waters ...

Also: Bf4 is the Harrwitz attack - which I consider a sideline.