Dependent/Linked Drop Downs In Cells

Now he has another task that I am not having any success with. He has asked if we can make an Excel spreadsheet that allows us to access a list of customer units from a drop down menu which is in another workbook, and once you choose that particular unit, another drop down menu will allow only those customers who are assigned to that unit to be selected from that sub-menu. I hope that makes sense.

Say customer A1 (we will call him Delta Co, 2-1 Aviation) is selected from this first drop down menu (which I haven't figured out how to make yet either). Then this selection would access another worksheet in that other workbook with a new drop down menu with only the four customers in that unit. As an example we will use the names SSG Wilson, SGT Jackson, SFC Jones, and SGT Phillips.

So when the Supply Tech here opens the spreadsheet, the first drop down menu, he will select Delta Co, 2-1 Aviation. Then the next menu will only show four names, and he will select SFC Jones.

Once we get this going, I may be able to figure out the other things the commander wants, if not I will be back.