The mortgage is the security instrument. Some states call the security instrument a security deed, some a deed of trust. Some states call it a mortgage. By signing the mortgage she is not agreeing to make any payments, even in the event of default. At least by signing this document, she knows that her husband is purchasing a residence. There are some states that do not require this practice and husbands and wives can purchase property without the knowledge of the other spouse. If my spouse were purchasing a residence, I would certainly want to know.
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What you learn after you know it all is what counts.