S - Squares/Space. This gets back to Pawn Structure, but shifts the focus a bit, and is more complex in some sense. Oversimplified, extra space offers that side more maneuver room for pieces. However, hedgehog defenses prove that sometimes the pieces of the side with less space have tremendous latent energy, just waiting to explode if the side with more space lets up for an instant.
A simple system for evaluating each element of KIMPLODES
This week the S-factor: Space/Squares
Space/Squares: basic building blocks.
As pawns advance, they leave holes behind them, but they gain a spatial advantage, potentially limiting the number of moves available to the enemy forces. On the other hand, they may become more vulnerable as they advance, moving further from protecting pieces. Additionally, if the pawns were in front of their King, they may leave him barren and alone, drifting under the cold gaze of enemy forces unhindered by pawns in the way.
Numerous authors have written about the importance of key squares, and there are common attacking themes that look at the f2/f7, h2/h7 and even g2/g7 squares, respectively for each side. In numerous Sicilian lines there are common sacrificial themes on the e5, e6 and c3 squares, so players must be constantly aware of the state of those squares. Another famous type of square is the blockade square directly in front of a passed pawn, behind which a Knight often radiates significant power because no major pieces can attack it along the file behind the passed pawn.
In general, extensive study of space and squares is a more advanced topic, and one that can be neglected to a certain extent (though not completely) by players rated less than 1600. At that level it is more important to learn the simpler concepts of DEvelopment and Lines, and stop making simple tactical mistakes, than to worry about space or squares. But beyond that 1600 threshold, approximately, the S-factor exerts an ever increasing influence.
Space/Squares in the opening
There are openings in which one side attempts to dominate space on one side or the other. In Alekine's Defense (1 e4, Nf6) the Four Pawn's Attack was an early attempt to refute the hypermodern child of Alekhine's imagination. In other openings, both sides attempt to grab space on opposite wings. For example, the Kings's Indian Defense, Bayonet version below sees White grabbing space on the Q-side and Black on the K-side...naturally their attacks will occur on those respective sides.
Space/Squares in the middle game
This is a beautiful game played by Vasily Smyslov against Jaan Ehlvest, in which Smyslov's entire energies are focused around the e5 square. Beautiful play, a pleasure to watch.
A more famous example is game 16 between Karpov-Kasparov in their WCh match in 1985. Kasparov introduced a gambit with 8 ... d5!? during this match, and in this game a very significant portion of the play is focused around the d3 square, with the closest enemy or friendly pawns at least 2 squares away in any direction. Fascinating stuff.
...and "That's all--the end is HERE!"