Changes

The first two options are probably easier to implement. They are usually stored as text files, such as files of PGN format (for openings lines) or EDP format (for last positions). The most advantage is that by using text form they are readable, understandable and editable by humanwithout any special software. However, the main disadvantage is that they may take too much space as well as time for searchingsince an item may take from 50 bytes (for FEN strings) to hundreds of bytes (for PGN text). If we store openings "continuously" (all possible opening positions) both data size and searching time become too hugeand become unacceptable for playing. In practice, those opening books are usually very small with some lines or positions only. They , they cannot be used for real game playing but for testing.

The third has some important advantages, especially for computer chess engines. Opening databases built on all positions are usually more space efficient and overcome the problem of [[Transposition|transpositions]]. Often these positions are stored as hash value to allow fast access. Some opening database formats store with each position as well some information about it. (e.g. number of times occurred, number of games won by white/black / drawn with this position, average/maximum Elo of players playing to this opening position, chess program's success with the position). So when a chess program has to decide on a move, it will look up all positions that could arise after moving in the database and then make a decision upon the information given with each position. Depends on designs, an item may take 10 - 16 bytes. To build and/or edit those opening books, users need to use special software.