From my first use of FM, I always defined an autoentered serial n

It coud be because your using number fields somewhere along the way. Or some number-only function that strips text, like Max() or Sum()....

Yes Eric I do now use Max and others.

That is why I ask if there is a rule to use numbers or text fields. But if I use a text field, how would the invoices be sorted...

Is it the common rule to add text into the serial N

Instead of using the formula:

Max(selfjoin::SerialNumber)+1

... you can generate new serial numbers with:

"VEND"&(Max(selfjoin::SerialNumber)+1)

to make sure that the prefix remains.

There was a solution obviously. Thank you muchy.

Nope ! Doesn't work for me

I might be missing something, but it seems to me that this would cause a sorting problem (as you expressed concern over), when you go from 9999 to 10000 for example. To avoid this, I'd start the serial number at a higher number like 1000000 (assuming you never expect to have more than 9 million records).