Public variable in private routine across worksheets

I'm using a double click macro to navigate between sheets but I would like to set up a variable which can be used to detect and return to the original sheet (like a "go back") button.

I have used code on each of the sheets which starts:

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)

but I can't find a way to declare a variable (e.g. "originalsheet") which is accessible across all the sheets.

Having looked around, I understand Dim only works in the same macro or module, so I've tried Public and Static but couldn't get these to do what I wanted/expected.

Is this possible?

Thanks!