Question : Excel VBA automate sequential numbering.

How do I automate a sequential number concatenated with two alphabets in the second column of each row i.e. AF001 ( in row 9, col b), AF002 (in row 10, col b)  AF003 (in row 11, col b) and so forth.  I only want the ID to appear when they double click on the cell i.e.  if they double click in row 9, col b, then place the value AF001, if they double click in row 10, col b then place in the value AF002 and so forth.  Please note I need step by step like where to put it in the vb editor.
Background information:
I only need this for one or two sheets (not all the sheets) in the entire workbook, not at the workbook level, but the work sheet level.
Thanks.

Answer : Excel VBA automate sequential numbering.

It was working perfectly in my test workbook (and still works now) ... but there was a problem with the ActiveSheet.Names references. I also tried using the sub in more than one worksheet and found that it wouldn't increment. To address this issue I suggest the following two subs (one for Sheet1 and the other for other worksheets). You only need to create the SerialNumber constant in Sheet1 (using Insert...Define...Name). The two subs will share a common base for serial numbers, each one remembering the latest value.

For Sheet1:

Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim SerialNum As Integer, temp As String
If Target.Column <> 2 Then Exit Sub
temp = ThisWorkbook.Names("SerialNumber").RefersTo
temp = Right(temp, Len(temp) - 1)
SerialNumber = Val(temp)
SerialNum = SerialNumber + 1
temp = "=" & SerialNum
ThisWorkbook.Names.Add Name:="SerialNumber", RefersToR1C1:=temp
Target = "AF" & Format(SerialNum, "00#")
Target.Offset(1, 0).Select
End Sub


For other worksheets

Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim SerialNum As Integer, temp As String
If Target.Column <> 2 Then Exit Sub
temp = ThisWorkbook.Names("SerialNumber").RefersTo
temp = Right(temp, Len(temp) - 1)
SerialNumber = Val(temp)
SerialNum = SerialNumber + 1
temp = "=" & SerialNum
ThisWorkbook.Names.Add Name:="Sheet1!SerialNumber", RefersToR1C1:=temp
Target = "AF" & Format(SerialNum, "00#")
Target.Offset(1, 0).Select
End Sub


If the worksheets will be using different serial numbering bases, the just make multiple copies of the first sub, each with its own serial number constant (you'll need to use unique names--such as SerialNumber1, SerialNumber2, etc. for the various sheets).