Question

Mattie J.T. Stepanek
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I think J.T. is Mattie's middle name? Would you let me know the full form of "J.T."?

Answer 1

Thnak you. I found the answer on the Internet.
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Matthew Joseph Thaddeus Stepanek (July 17, 1990 – June 22, 2004), known as Mattie J.T. Stepanek, was an American poet who published five best-selling books of poetry.[1] Before his death (at the age of 13) he had become known as a peace advocate and motivational speaker.

Answer 2

Matthew Joseph Thaddeus Stepanek
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Matthew => first name
Joseph Thaddeus => middle name
Stepanek => last name

(Is my explanation correct? Why are there two words in the middle name?)

Answer 3

Q: Is Mattie the petname of Matthew?

Answer 4

This man had two middle names. The first and the two middle names are also referred to as <i>given names.</i> The last name is also referred to as his <i>surname.</i>

And yes, Mattie would be a pet name or a nickname for Matthew.