According to tradition, the qin originally had five strings, representing the five elements of metal, wood, water, fire and earth. Later, in the Zhou dynasty, Zhou Wen Wang 周文王 added a sixth string to mourn his son, Bo Yihou 伯邑考. His successor, Zhou Wu Wang 周武王, added a seventh string to motivate his troops into battle with the Shang. The thirteen "hui" 『徽』 on the surface represent the 13 months of the year (the extra 13th is the 'leap month' in the lunar calendar). The surface board is round to represent Heaven and the bottom board flat to represent earth. The entire length of the qin (in Chinese measurements) is 3 feet, 6.5 inches, representing the 365 days of the year (though this is just a standard since qins can be shorter or longer depending on the period's measurement standard or the maker's preference). Each part of the qin has meaning, some more obvious, like "dragon pool" 『龍池』 and "phoenix pond" 『鳳沼』.