Irregular

By default, all of the sides of the polygon are equal in length (eg, a 3-sided shape is always an equilateral triangle, a 4-sided shape is always a square, etc). Check this box to make it possible to create "skewed" shapes with sides of different lengths - rectangles instead of squares, ellipses instead of circles.

Tip: This setting can have strange interactions with certain "Snap Angle" values - if you need to create an irregular shape, it's generally easiest to set the snap angle to 0, drag out the shape, and then rotate it as needed with the "Rotate Shape" tool.