Across, crosswise, crossways and athwart all are synonymous when they mean so as to intersect the length of something.
Across and athwart may be used as prepositions as well as adverbs but carry the same implications in either part of speech.
Across usually implies extension or passage from one side to the other.
- this board will not go across
- he could not get across the river that night
Crosswise and crossways stress intersection at right angles and usually suggest a horizontal direction.
- the stripes run crosswise
- the defect lies crossways to the grain of the wood
Athwart commonly implies obliquity of direction or intersection at an acute angle.
- the tree fell athwart the road
- on the slopes the shadows lie athwart
- in some weaves the filling threads run athwart those of the warp
Figuratively, especially with reference to plans, purposes, hopes, across and crosswise are not always synonymous because they retain and stress their distinguishing implications.
Across often implies fulfillment.
- he was able to get his point across to his audience
While crosswise implies contrariety and therefore frustration.
- everything goes crosswise with us tonight