The name Semi-Slav suggests that somehow it's half of a Slav. How did this name come about? Or what does it mean? The two openings seem completely different to me due to the fate of the c8 bishop.
1 Answer
It simply because
With Black advancing pawns to both e6 and c6, the opening resembles a mixture of the Orthodox Queen's Gambit Declined (QGD) and the Slav Defense.
(from wikipedia)
If you consider that there are two components to the mainline Slav - protect the d5 pawn with c6 and then develop the light-squared bishop to f5/g4 - then the semi-Slav only shares one of these two features.
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I would like to add to this. Semi Slav structures are transposed a lot from other openings. It is one of the most flexible setups for the black side. Commented Sep 12, 2021 at 18:09