Understanding Vector Notation: Solving Homework Equations

[superg33k]

Homework Statement

Does
$$r_{12}$$
mean distance from point 1 to point 2, or point 2 to point 1?

Homework Equations

The Attempt at a Solution

 

[RTW69]

Where you typically see this notation is the vector form of Coulomb's law where electric force exerted by a charge q₁ on a second charge q₂ is written F₁₂

 

[superg33k]

Cheers, I'm using in it Biot-Savat law so same thing really. I always forget it, it drives me nuts! Thanks again.