# In 1D why are vectors, such as, e.g., velocity $v$ and acceleration $a$, not written with a vector arrow?

In my textbook instantaneous velocity and acceleration are wrote only as $v$ and $a$ in the chapter of One dimensional motion. Instead, in two dimensional motion chapter they are written with the "arrow" symbol upon them, $\vec{v}$ and $\vec{a}$.

Why if they are both vectors they don't have an arrow in that particular chapter? I just can't get it.

Having an arrow on top of any symbol means the quantity is a vector quantity, which has both magnitude and direction. $\vec{v}$ means the velocity vector, while $v$ Or $|\vec{v}|$ means the magnitude of the velocity vector.