This is a soft question.
Context: In mathematics, one often writes in boldface to signify certain mathematical objects. For example, we might write $c$ to signify a real number, and yet write $\mathbf{c}$ to signify a column vector of real numbers.
The problem is that it is hard to write $\mathbf{c}$ on paper or a chalkboard/whiteboard; writing $\mathbf{c}$ on paper just looks like writing $c$.
Of course, we never run into this problem on math.stackexchange because we write in latex.
Question: Is there some widely held convention in the mathematics community that allows one to signify bold letters when writing on paper (or chalkboard/whiteboard)? Underlining? Underlining twice, or some such practice?
The standard marking, in the sense of standard proof correction marks, for boldface is to underline with a wavy line. (A straight underline is for italic, and underlining twice is for small caps.)
While this is standard in a way I doubt this is all that widely known nowadays (I'll confess I had to look it up to recall which type of line it was).
There is also the font-family called blackboard bold that you surely know $\mathbb{A,B,C,N,Z,Q,R}$ hinting at another way to write bold on the blackboard, namely draw some lines twice.
For practical purposes, I suggest to do whatever works well for you. Likely underlining with a straight line is preferable to the 'standard' wavy line in this context, as likely you will not have the need to also signal italic.