Note. Strange behavior of excel. Has anyone dealt with it and can explain this strange phenomenon? . I've included excel files for my post. They're on github. One is myCellEx.xlsx excel file, the other is sheet.xml after unpacking it https://github.com/PiotrJanosik/excel As you know, excel is a package of zipped files. Unpack myCellEx.xlsx and see the entries in the sheet.xml file. The column named B @ Z1 has the value: 1378. There is no such column !. There should be a column named AZZ1. The excel program should signal an error when it opens a file in which the wrong cell address is entered because there is no such cell address B @ Z1. See that I generated an incorrect sheet.xml file in which the wrong cell address format is stored and the excel program opens it. I wrote this address on purpose .. Anyone know why excel allows it? .I'm learning programming.I'm experimenting with Open XML. Therefore, I found it incomprehensible behavior of excel.