How to represent a tensor/matrix/vector/array in blackboard? While teaching in the blackboard, I find it difficult to represent a vector/matrix/tensor. In latex, we represent $\mathbf{x}$. But, how do we represent it while writing in chalk? Is it okay to write an underlined variable $\underline{x}$ instead? What is the usual practice?

I think, avoiding the bold text might confuse the students.
The following are the notations I've seen most along the years (I don't think there's a winner). 

Vectors:



Matrices:



In my experience, unadorned symbols are preferred by mathematicians; the arrow is preferred by physicists; and bar and underbar (and double underbar for matrices) are preferred by engineers.

Even though I don't have to write many vectors and matrices (in my classes I deal mainly with scalar quantities), I usually employ underbarred symbols for four reasons: 


Of course, I'm an engineer! 
A bar is faster to draw then an arrow.
My handwriting is awful, and I think that simpler symbols improve readability; at the same time, though, I don't want to abandon the categorization of quantities through different symbols.
The interpretation of underbar is frequently that of bold, and I prefer bold symbols for vectors over symbols with an arrow.