What are these two 'x's in threadIdx.x?

I was just curious about the two 'x’s in threadIdx.x. As one can see that blockDim.x has only one ‘x’ for denoting x component. But, what is the significance of the first ‘x’ in the threadIdx.x? Maybe somebody from CUDA core team member might tell us.

Thank you.

– Mandar Gurav

Idx comes from shortening index .x is the first element of the structure, nothing special there.