This is a slash: /
This is a backslash: \
The “no” sign (as in “No Smoking” or “No Left Turn”) is a circle with a backslash.
For some unfathomable reason, almost all of the amateur “no” signs I see use forward slashes, even though almost all of the professional “no” signs I see use backslashes.
Note: This is the situation as I understand it in the United States. I'm told that the conventions are different in other parts of the world.