someone should make a sticky to summarize this
then can be use as a reference thread in case question comes back in future
happy bl
Iāve done some digging and found that Blender is a y-up system. Have a look at:
- the Camera Track To constraint,
- the UV Editor,
- the default Bone orientation,
- Pose mode,
- the compositor,
- image editor,
- video sequencer,
- movie clip editor, and
- hair duplication.
The Z-axis is only āupā in one place: the 3D Viewport.
Wow I just read through this thread and only at the end realized that it was from 2015 
:
Anyways, of course Y is up in all cases where weāre working in 2D, as there is no Z axis at all. So: camera view, all image editors, movie clip editor, sequencer, etc.
The camera track to constraint has Y-up, as it refers to the axis of the 2D camera view.
Now when we have 3D space, like the viewport, then we have another axis (Z), and this one becomes up. So basically in every Nth dimensional space, the Nth axis would be āupā. That seems logical to me.
I cannot however explain bone orientation. And about hair I canāt say anything as I donāt know or use that feature.
So it seems pretty logical to me.
Now when we have 3D space, like the viewport, then we have another axis (Z), and this one becomes up.
Yup, it becomes up, as you say. In other words, we reorient ourselves from one system to the other.
Iāve been pondering all this lately and realized that z-up is used by mathematicians and architects, but not artists. I guess that means Blender, like 3DS Max, is primarily an architectās tool.
Thatās pretty easy: itās not āupā. Itās āalong the boneā. Forward/back, if you will, which is, incidentally, Blenderās axis for typical object orientation as well.
Wow. I didnāt realize objects were y-up, too. Thanks for pointing this out.
Has nothing to do with either. When you look at a piece of paper on a table, youāre looking down on it, and have your X and Y axes. Same happens in Blender in top view, when youāre looking down on the grid.
In either case, Y is āupā in the canvas, but not in the world. In the world, youāre looking down along another axis, which happens to be Z.
Yup. Architects look down on plans, but animators and filmmakers look at screens which are oriented x/y.
But if youāre an architect, I can see your point and youāre right. Z is up for architects. I know this for a fact because I checked with a friend of mine this morning. Heās been an architect all his life (heās 60 now, so thatās quite a while).
Iām not arguing. Iām just trying to understand the logic of Blenderās orientation.
I just thought of something elseā¦
From a birdās eye view point of view (like an architect), the birdās bonesā y-axes would be mostly oriented horizontally in flight (a birdās most natural orientation) meaning a birdās bones would have the z-axis lying horizontallyā¦
While from a human characterās point of view (like an animatorās or filmmakerās) the bonesā y-axes are mostly oriented vertically while in an upright posture (humans are considered the quintessential upright ape) which means the y-axis is vertical.
Funny how the two POVs are oriented one way while the bones are oriented the other.
With this logic, you also needed to reorient yourself to the Y-up in 2D, since in 1D-space we have X-āupā 
Yes I think that is a good way to summarize it. Not only architecture, but engineering in general. And Iām glad that weāre following standard mathematics, including a right-handed coordinate system (some Y-up software use left-handed systems).
Youāve lost me. Where is this one-dimensional space of which you speak? 
Kind of leaves animators and artists out, though. Kind of shame since art-oriented users are purportedly the ones BI seems to be trying to pull in as new users. Ah, well. Itās their pony ride.