Queen's Gambit Declined

Queen's Gambit Declined (or QGD) is a chess opening in which White offers a gambit of a pawn (the Queen's Gambit), but Black then declines to take the offered pawn in order to avoid giving up the center of the board:

1. d4 d5
2. c4 e6

The Orthodox Defense (or Orthodox Line) is: 1. d4 d5
2. c4 e6
3. Nc3 Nf6
4. Bg5 Be7
5. e3 O-O
6. Nf3 Nbd7
7. Rc1 ...

Similar to the Orthodox Defense, the most popular way to decline the Queen's Gambit is the Ragozin Variation, which is similar to Nimzo-Indian Defense:

1.d4 d5
2.c4 e6
3.Nc3 Nf6
4. Nf3 Bb4

This can continue as the Vienna Variation:

5. Bg5 dxc4
6. e4 ....

Other Defenses

• 1. d4 d5 2. c4 e5 – Albin Countergambit
• 1. d4 d5 2. c4 Nc6 – Chigorin Defense
• 1. d4 d5 2. c4 Bf5 – Baltic Defense
• 1. d4 d5 2. c4 Nf6 – Marshall Defense
• 1. d4 d5 2. c4 c6 – Slav Defense
• 1. d4 d5 2. c4 e6 and subsequently ... c6 – Semi-Slav Defense
• 1. d4 d5 2. c4 c5 – Austrian Defense (also known the Symmetrical Defense)

https://www.chessgames.com/perl/chessopening?eco=D63

https://simplifychess.com/queens-gambit-declined/index.html#what-is-the-queens-gambit-declined