John, If you refer your customer to the equations for stripline and microstrip (or better show a drawing), hopefully, it'll become clear that it's ALWAYS THE CASE. Examine the equations to find out how the dielectric "constant" and distance..etc would affect the impedence. A good reference that I've used is Motorola's MECL design databook. The last one I looked at was a 1989 book. Not sure if there is a newer version!! If anyone out there knows of a newer version, please LET ME KNOW. Thanks. Groovy got me hooked on this book a while back. thanks Groovy. bilal khalaf >This controlled impedance stuff is new to us... > >All the reading I have done seems to imply that reference planes are >adjacent to the signal plane. Is this typically the case, or always the >case? >I have a customer who keeps submitting designs which call for a controlled >impedance line where the reference plane is not adjacent. > >Any input in helping us understand this would be appreciated. > >Regards