Why do people draw a horizontal line through the number seven and when is it appropriate to do so?

Hi,
This comes from the Z, which many draw a line through the letter Z, to keep it from looking like the number 2.
A 7 is the same principle (although it doesn't look like a 2), and you can draw a line through it if you wish, anytime.
Best wishes.

Drawing the line through the seven is a french custom. In the same way that their long division looks different then ours, so does the number 7.

Interesting! I have always put a line through my 7's. I also put a line through my Z's, both upper and lowercase. I've never really thought about why.