I'm a little confused about something in projectile motion.

In a revision book I have, a person throws a dart horizontally, and you have to work out distance, time taken to hit the ground etc.

I understand no problem that you take horizontal and vertical motion separately. What I don't get is why is acceleration (in this case g = 9.81) for the vertical component negative? Surely it is accelerating towards earth, so 'a' should be +9.81?

The final distance it travelled vertically was given as a negative value as well, but I think I understand this - it's because the dart ends up below the starting position. The only problem I have is with negative acceleration.

Thanks in advance!