One of the things that my students get frustrated with when they are making drawings is how CATIA will switch the direction of the dimension you are making based upon where you are moving your mouse.
This can be especially annoying when doing a chained, cumulative, or stacked dimension. This can be controlled 2 different ways. The first way is to choose an option in the tools palette like the force horizontal or force vertical dimension.
The other way is to hold down the Ctrl key to lock the current dimension direction.
This topic is covered in more detail in the Rand 3D training class, CATIA V5: Generative Drafting.
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