# What does the “$T$” superscript mean on vector?

My relativity book defines the "worldline" of a system as:

$$$$x(\tau)=(x^0(\tau),x^1(\tau),x^2(\tau),x^3(\tau))^T$$$$

I often see velocities written in the same form: $$U=(0,u^2,0)^T$$

What does the "$$T$$" superscript mean?

The $$T$$ stands for "transposed". The vectors $$x(\tau)$$ and $$U$$ are column vectors, but they are printed as transposed row vectors to save space.
• So that's why the superindex is removed once the vector is expressed as $\begin{pmatrix} x^0(\tau) \\ x^i(\tau) \end{pmatrix}$ – IchVerloren Feb 21 at 2:09