Falling Diagonal Crossing Rising Diagonal

A crossing where the falling line passes over the rising one.

Look closely and ⤬ is not a plain X. The line running from top-left to bottom-right (the “falling diagonal”) is continuous. The line from bottom-left to top-right (the “rising diagonal”) stops short of it on both sides, leaving a small gap. There are no arrowheads; the name just describes the drawing: a falling diagonal crossing over a rising one.

That gap is borrowed from knot theory, the branch of mathematics that studies knots as closed loops of string, including the plain untangled loop. Mathematicians draw a knot flat on the page as a knot diagram: a loop that crosses itself in places. At each crossing one strand is in front and one behind, and the one behind is drawn with a break in it. Every knot diagram is just a loop plus a choice of over or under at each crossing, so a symbol for “falling strand over rising strand” and its mirror image ⤫ (rising strand over falling) give a writer a way to name a crossing type in running text without drawing a picture.

Where will you meet it, and will it display?

In maths writing, and sometimes as a blank box elsewhere.

⤬ is a specialist character from books and papers on knots and related tangled-string shapes. If one turned up in a message or a name, the sender may simply have wanted a stylish X, since the silhouette reads as an X at a glance. The catch is that not every font includes it, so the person on the other end may see an empty rectangle instead. If you want an X that displays almost everywhere, use the letter X or the multiplication sign ×; use ⤬ only when the over-under meaning is the point.