What is this notation?

Question

I don't recognize the notation used for the $x_i$ directions below (passage from textbook).

The components of a vector depend on the base vectors used to describe the components. This will also be true for tensors.

Let $\hat{e_1}, \hat{e_2}, \hat{e_3}$ be unit vectors in the direction of $x_{1^-}, x_{2^-}, x_{3^-}$, respectively, of a rectangular Cartesian coordinate system. Under the transformation $\boldsymbol{T}$, these vectors $\hat{e_1}, \hat{e_2}, \hat{e_3}$ become $\boldsymbol{T}\hat{e_1}, \boldsymbol{T}\hat{e_2}, \boldsymbol{T}\hat{e_3}$. Each of these can be written...

The book goes on without mentioning the notation anywhere nearby. And this is the first mention in the book.

Answer 1

He writes phrases like, "... about the $x_3$-axis... " quite a bit I'm seeing. So, in context, I'm gathering he mean axis by the dash symbol. That is, it's not a minus sign.

Answer 2

Ok, from the reference you gave me, I found an excerpt on google books.

In this version of the book, the author says $x_1$-,$x_2$-,$x_3$-axes so it is effectively the standard vectors $(1,0,0),(0,1,0),(0,0,1)$.

Although there are not hats on the $e_i$ in this third edition. It seems this part has been changed in fourth edition and they omit the word axes, probably an impression mistake.