Opening Classification :
The ECCO Classification

Introduction to ECCO
This is the first and only (thus far) system for Xiangqi openings that I have found. 

http://www.xqbase.com/ecco/ecco_intro.htm is the url but it is entirely in Chinese. They seem to have an English function located somewhere, but I do not think that the English is entirely friendly itself...
It first appeared in 2004. I do not know who created it but it seems that it was created to help categorize openings for
www.xqbase.com . Currently, the major Xiangqi websites and engines all adhere to this classification.
ECCO is the acronym for Encyclopedia of Chinese Chess Opening and is a reflection of contemporary Xiangqi openings.
Some of its merits include the ability to categorize the less seen openings like P1+1 or A4+5 while at the same time being able to list the commonly used openings and various variations like the central cannon vs screen horse defense with edge cannon for chariot exchange variation.
According to information given on the site, it was initially called ECCO 2004 as there were 8197 games collected and categorized by the authors up till 2004.
Traditionally, “orthodox” Xiangqi openings/counters were simply classified as one of the following:
a) cannon openings (central cannon, palcorner cannon, cross palace cannon)
b) elephant openings
c) horse openings
d) pawn openings.
“Unorthodox” openings like the single horned monster 独角兽, the nine-tailed tortoise 九尾龟 , tandem cannons counter 重疊炮, golden hooked cannons 金钩炮, phoenix cannons counter凤凰炮 , the turtle back cannons 龟背炮 etc… were unclassifiable. ECCO has been able to overcome this situation.

Before you continue, I have "invented" a shorthand version of the openings so that I can classify my stuff much more neatly. Click here ...

How the ECCO label is formed:

The ECCO label is arrived based on the following criteria.

a) The order of the moves.

Based on the “opening tree” (that is, the algorithm you get when you take into consideration ALL the possible opening moves), most NON-Central Cannon openings will arrive at some common formation. This formation would be then given an ECCO label. For example, A40 ( a variation of the horse opening) would refer to the same formation arrived even though the pawn opening may be played first. 

b) The process of making the moves.

  The order of first few moves that are played before a predetermined formation is arrived may not influence the final formation. This has been a problem for the authors of the classification system but generally, as long as the moves are played in logical order and can arrive at the same formation, it would be given the same label.

For example, in the double headed snake pawn formation, it would not matter much if P3+1 is played before P7+1 or vice versa, and as long as both moves are played, it would arrive at the double headed snake pawn formation.

To further validate this point, D28 is given as the label to represent the following opening, “Same Direction Cannons: Filed chariot with Double headed snake pawn formation vs Ranked Chariot. It would not matter if red played P3+1 or P7+1 as long as both moves are played, it would be the double headed snake pawn formation.  

c) Formations occurring with high frequencies:

In openings where it is very commonly played, the classification becomes trickier. Basically, the authors have determined which pre-determined formation is reached first and the moves that follow would be designated as its variation.

The Central Cannon vs Screen Horse Defense is THE most played opening. In such an instance, ECCO would have labeled many different common formations. It takes into consideration that the same formation CAN and WILL be arrived from a different order of moves. For example, in perhaps the most commonly used opening, the Central cannon 7th Pawn Advancement with Pawn Ranked Chariot vs Screen Horse Defense 7th Pawn Advancement is designated as C30-C49. There are 24 possibilities for the order of moves to arrive at this formation. In such an instance, the authors have recognized the major formation and the rest of the moves will be the variation or sub-variations of this opening. Therefore, if black played the Edge Cannon for Chariot Exchange variation, it would be labeled as C49 and is considered to be a variation of the Central Cannon 7th Pawn Advancement with Pawn ranked Chariot vs Screen Horse Defense with 7th Pawn advancement and Edge Cannon for Chariot Exchange variation.  

Basic ECCO classification :
There are basically 5 broad categories to ECCO. The moves required to generate the code are given in the respective sections below.
A) Non-central cannon openings (not inclusive of the Angle Pawn) : basically inclusive of the Elephant opening, Horse Opening, Palcorner Cannon Opening, Cross Palace Cannon Opening and other less commonly used openings like P1+1/C2=3 …非中炮类开局(不包括仙人指路局)
     A00-A3x  A4x-A6x  
B) Central cannon vs Sandwiched Horse Defense 中炮对反宫马及其他
     B00-B2x  B3x-B5x  
C) Central cannon vs Screen Horse Defense 中炮对屏风马
     C00-C2x  C3x-C4x  C5x-C7x  C8x-C9x   
D) Same direction cannons and Opposite direction cannons (inclusive of the halfway opposite direction cannons) 顺炮局和列炮局(包括半途列炮局)
     D0x-D2x  D3x-D5x  
E) Angel’s Pawn Openings 仙人指路局 
    
     E0x-E1x  E2-E3x   E4x  

Each opening and counter is basically represented by a capital letter (A-E) followed by a two digit number. For example, D28 would refer to "Same Direction Cannons: filed chariot with double headed snake pawn formation vs ranked chariot".

Other points to note:
ECCO does not take into account "mirror images". The defaults are chosen in determining the label. For example, by default, the Elephant opening vs Left Central Cannon Opening occurs when Red plays E3+5 and black answers with C8=5. This is labeled A27. If in an actual game, red plays E7+5 and black answers with C2=5 ,this opening would still be labeled as A27, despite the right cannon being played instead of the left cannon. This is to take into account the RELATIVE positions of the pieces played in the opening.

First Created: 25th Nov 2011
Last updated: 28th Oct 2012
Acknowledgments:
1. Georg Jeiter from Germany has proofread this page and a few typing mistakes have been corrected. Thanks!
References:
1. the website whose url is given above

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