If a system requires that commands or other information be entered as upper-case or lower-case characters only, it is considered to be case-sensitive. For example, the commands format and FORMAT may seem identical to a human being, however a computer may only understand one of these as a valid command. Likewise, some operating systems may treat filenames such as Report.txt and report.txt as different files. It is important to consider this aspect when troubleshooting a system.