In Daniel 5:7, Belshazzar promised that whosoever read and interpreted the handwriting on the wall would be made the third ruler of his kingdom. This promise was fulfilled in verse 29 when Daniel was proclaimed the third ruler after reading and interpreting the said writing. I find it difficult to understand why third ruler. If Belshazzar was the king at that time and the Bible did not mention that he already had a second ruler, why not make Daniel the second ruler instead?
Nabonidus The King of Babylon (1st ruler) during the events of Daniel Ch. 5 had engaged Cyrus’ forces in battle but was Nabonidus took refuge in Borsippa (SSW of Babylon) and he left his Son, Belshazzar, to rule while he was engaging Cyrus, thus Belshazzar was acting as King Regent (2nd ruler), thus he could only offer Daniel 3rd place in rulership.