A 2-lead crystal or resonator can be rotated in a circuit, as it does not have a polarity.
Crystals and resonators with more than two leads can sometimes be rotated 180 Degrees, but sometimes not. Checking the schematic in the datasheet is the only way to find out.
Example of a Crystal Schematic that Can Be Rotated 180 Degrees
-Notice the two leads of the crystal will land on the same pads at 0 Degrees and 180 Degrees, and the other two pins can be flipped also since they are both ground. The index (cutout) in the one corner pad for this particular part number is used only for orientation reference so the part isn’t placed at 90 Degrees or 270 Degrees, but has no further significance.
Example of a Crystal Schematic that Cannot Be Rotated
-Notice the leads will not land on the same pads at any degree of rotation:
Oscillators Cannot Be Rotated
-Notice the leads will not land on the same pads at any degree of rotation, since each pin has a unique function:
See Also
Crystal & Resonator Differences
Summary
orientation
oriented