1. Adjustable headgear which comprises: an occipital strap having an upper end and a lower end adapted to extend from the back of the head of the wearer upward to the top area of the head; a right side strap having an upper end and a lower end adapted to extend from the right side of the head of the wearer upward to the top area of the head; a left-side strap having an upper end and a lower end adapted to extend from the left side of the head of the wearer upward to the top area of the head; a top adjustment piece formed of flexible plastic material or the like and having a first pair of slits extending generally parallel to one another adjacent the left side thereof and a second pair of slits extending generally parallel to one another and to the slits of the first pair adjacent the right side thereof, the first pair defining a left-side loop and the second pair defining a right side loop of the material, and a third pair of slits extending crosswise of the top adjustment piece defining a crosswise loop, the upper end portion of the left-side strap extending across said adjustment piece from left to right through the left-side loop and then through the right side loop, and the upper end portion of the right side strap extending across said adjustment piece from right to left through the right side loop and then through the left-side loop, the upper end portion of the occipital strap extending through said crosswise loop between the left-side and right side loops; and backstrap means having forward end portions connected to the lower ends of the right side and left-side straps and extending around from the lower end of the right side strap to the lower end of the left-side strap, the lower end of the occipital strap being connected to the backstrap means generally centrally of the latter.