Structured exception handling is based around the idea that while exceptions should be used for unexpected conditions, they can be built within your application structure. Some older languages would allow for generic error handling that didn't exist within a defined set of boundaries. However, professional developers learned long ago that even unexpected conditions should be definable within your application structure.
To allow for this you may have what is known as a last-chance error handler at the topmost level of your application; however, most error handling is structured within individual modules. Within Visual Basic error handling depends on four keywords. Three of these are associated with properly identifying and handling exceptions, while the fourth is used when you wish to signal that an unexpected condition has occurred.