Why direction of axes of coordinate often not indicated in diagram?

Is there any case/reason of not showing the directions (from negative to positive) of axes in coordinate diagrams? Sometimes I see the direction not specified, even though the discussion using that diagram assumes certain direction, which makes me confused a lot of times.

I thought it may be better to give an example domain to consider cases/reasons. In my dynamics class, I've never seen any coordinate diagrams, either 2D or 3D, with directions indicated.

Thanks.

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Indicating directions is very high school. There are standard conventions, so there is no point to indicating directions. – André Nicolas Apr 8 '12 at 1:44
Thanks for the comment. Can I ask what is "standard conventions" you're talking about? – IsaacS Apr 8 '12 at 1:48
And this is just my partial experience, but my previous mentor who is a computer scientist (who got PhD at one of the top school in Japan) always indicated the directions. – IsaacS Apr 8 '12 at 1:50
You are of course familiar with right $+$, up $+$. Then for three dimensions there is the right-hand rule. – André Nicolas Apr 8 '12 at 1:51
@André Nicolas I see. Then I might only be experiencing strange cases with my dynamics textbook, where so often times "right +, up +" and RH-rule doesn't hold in the given diagram. – IsaacS Apr 8 '12 at 1:58