Use the adverb athwart to describe something that reaches or stretches across, or from one side to another. You might hang a "Happy Birthday" banner athwart your office doorway to celebrate your coworker's birthday.
You can describe things that stretch sideways or span between objects as athwart, and you can also use the word to mean "in opposition to" or "against." For example, you might say that your plans to paint the house dark gray run athwart your roommate's plan, which involves hot pink with gold trim. In the fifteenth century, the word was coined by adding a to the word thwart, which comes from the Old Norse root word þvert, or "across."