Former chess world champion Max Euwe supposedly said "strategy requires thought, tactics requires observation." But what exactly are we supposed to look for in a tactical position? And why does it not require conscious thinking? (At least not for chess masters) An important concept for solving tactical chess problems is pattern recognition. Great differences exist between chess masters and beginners (or even relatively accomplished amateurs) in the way they perceive a chess position within the first few seconds after seeing the board or after an opponent's move. Chess masters are extremely good at recognizing pattern almost instantly on the chess board. But what is a "pattern" and how is it stored in memory? This post attempts to explain the concepts of "chunking" and "pattern recognition".
Most models for the storage of complex pattern in memory are based on the so-called chunking theory. The concept of chunking during reconstruction of chess positions is based on psychological studies where both chess masters and amateurs were briefly shown a chess position and then asked to reconstruct it. Instead of placing one piece after another on the board, experienced chess players reconstruct a given position by quickly placing groups of pieces on the board, interrupted by some thinking time between the groups. These piece groups, or "chunks" have been considered to be storage units in memory.
One popular example for chunking is memorizing numbers with multiple digits. It turns out that the average untrained person is able to recall a number with up to 7 digits. However, when an individual is given a group of familiar numbers, let's say phone numbers of friends or family, he/she can remember about 7 different numbers. At first, this may sound strange because 7 phone numbers contain many more digits than the 7 digits the same person was able to recall before. However, the person does not need to memorize each digit of the phone numbers separately. A familiar phone number forms a chunk in long-term memory labeled with, e.g. "Fred", and all that the person has to remember is a list of 7 friends instead of a long chain of individual digits.
" by Dennis H. Holding. Several tactical
opportunities are hidden in this position (which was supposedly taken
from one of Fred Reinfeld's books): First, assuming that black is to
move, white's queen is 
en prise. If white is on move,
1. Rxd5 Qxd5 2. Re8+ Rxe8 3. Qxd5 wins the exchange.
Therefore, if the chunks used for reconstructing a position from
memory would be related to the tactics of the position, one would
expect that d1 and d5 would be in the same chunk (queen en
prise), or may be e5 and g8 (rook is able to check the black
king). In the example, however, the white queen is chunked together
with her own rook on e2, and that the important rook on e5 is not
chunked with any other piece. In the diagram pieces belonging to the
same chunk are marked with the same color. The chunk with both kings
(red) was placed first, followed by the important rook on e5 (orange).
Interestingly, the important groups with the white queen (purple) and
the black queen and rooks (dark blue) were placed last and second
last, respectively. These results show that the chunks used to store
a chess position in memory probably do not directly form the
pattern whose recognition triggers the correct move decision.
The main difference between chess masters and amateurs lies in the size
and number of chunks used to reproduce a position. Chess masters use
bigger chunks containing more pieces,particularly for middlegame positions,
and are also able to chunk more pieces. At the time the book "The
Psychology of Chess Skill
In summary, the brain seems to use several layers to reduce the
complexity of a chess position. First, experienced chess players do
not store the position of individual pieces but groups of pieces,
called "chunks", in memory. Second, the spatial relationship
between these chunks is used to associate the position with a pattern
stored in long-term memory which is then used as basis for the move
decision. Chess masters are using bigger chunks and have more pattern
stored in their long-term memory than amateurs. Therefore, they can
rely more on their long-term memory to make decisions. For a more
comprehensive description of the research on chess skills, see the
book "The
Psychology of Chess Skill
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written by CelticDeath, November 24, 2008
minor piece, i.e. Bishop or Knight. Therefore, in the problem shown, white wins the queen for a rook, but it is not said that white wins the exchange. 
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