If you're talking about direction:
* Widthwise: This is the most direct opposite, referring to the direction perpendicular to the length.
* Crosswise: This also implies a direction perpendicular to the length.
* Across: This can be used more generally to describe movement or orientation perpendicular to the length.
If you're talking about orientation:
* Crosswise: This implies a perpendicular orientation.
* Transverse: This is more formal and means "across or lying across."
* Vertical: This means standing upright and is opposite to lengthwise if lengthwise is considered horizontal.
If you're talking about cutting:
* Widthwise: This is the most direct opposite, referring to cutting perpendicular to the length.
* Across the grain: This refers to cutting perpendicular to the wood grain.
It's important to consider the context to determine the most appropriate opposite of lengthwise.