<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> David I'n not being antagonistic at all. I am just trying to help you understand something you seem clearly confused about. The two unknown formula link you offered for example.... there is no way you can use that in the context you have suggested.  Its a completely different problem type. The Tension formula is T = f^2*L^2*d^2* K  where K = the string density * PI / 981 You cant fit that into solving for an  aX+bY=c problem.  There is no way around it. You can calculate the change in length and measure the pitch change to get tension... or you can calculate change in length and change in tension to get pitch.... but you have to get two of these in order to get the third. As far as the diameter is concerned.  Use the spreadsheet I supplied with accompanying justifying formulas.  You can easily enough juggle the input tension for different wire diameters for same wire lengths to get the same starting frequencies. Then change the deflection input for both strings.  As long as they are the same length... any same deflection will cause the same frequency change. Cheers RicB <blockquote> See my other post but of course diameter plays a role.  It is a factor in determining break point percentage.  A thicker string will have a higher break point percentage and a thicker string will need to be at a higher tension to achieve a certain frequency at a given length than will a thinner string.  Simple stuff.  The claim about BPP as a factor in determining which string will go out of tune more goes way back.  You don't need to use BPP in the formula, you can simply calculate it for the two strings in question and observe the relationship-see my other (corrected post).  Please don't be so antagonistic.  I'm really trying to help you here.    </blockquote> </body> </html>