Claiming tax treaty benefits in Switzerland: the swiss only do it one way. You file the return as if it were a regular return, and concurrently file a request for refund.
This document gives the steps. The majority of the document speaks bout dividends and interest on retirement accounts but it is the same for regular interest. The document only has one paragraph devoted to teh general subject of tax treaty benefits.
The documents I use for reference are:
Please note: you will not find in irs code or the publications, the phrase "....principle is not taxed, or income already taxed is not taxed again". You will find that the term principle is not used in the context of taxable income.
Think of it this way. If the IRS could tax incomed twice, then it would be taxing your already taxed earnings no matter where you had it, in a bank account or under your mattress. Lets see how that works:
I take home a pay check, which has been taxed, and I decide to not spend it. Along comes the IRS and says, since you have not spent your money, I am going to tax it.....we both know this does not happen. Once your income has been taxed, and it is put into a savings account, the principle amount represents your earnings which were already taxed., Therefor only the interest or earned income is taxed.
The ONLY exception is that if you put your principle amount or already taxed income into an IRA account and take an early withdrawal, then you pay a 10% penalty for early withdrawal as well as taxes. But then, in a traditional IRA, the principle amount was not taxed in teh first place, so by taking it out, they are allowed to then tax it.
In a Roth IRA, the principle amount is after taxes, and so there for, the principle amount is not taxed again and the Roth IRA grows tax free, unless you take it out early and have to pay a penalty.
NOW the other exception to principle amount is if your principle amount ni your bank account in Switzerland represents some untaxed earned income.
My answers were based on the assumption, from informatlino in your question, that the money you placed in a swiss account to earn interest, was from your salary which wsas already taxed as part of payroll withholdilng or your payment of taxes with your tax returns; that this principle amount was take from payroll and then put in a bank account to bear interest, and that the principle amount had already been reported to the IRS when you filed your taxes.