Every Second Turn (also known as "Alternate Corners", "Every Second Breakpoint") is a rectangular or square grid with circles in some cells. The aim is to draw a single continuous non-intersecting loop that visits every cell exactly ones. It makes a 90 degrees turn at every cell with a circle. There is exactly one turn between two consecutive circles that the loop visits.
Cross+A can solve puzzles from 3 x 3 to 30 x 30.