Common Grass Yellow butterfly

The special feature of this type that is there are two spots, blotches at the base of the forewing. At the upper end of the front wing there are several variations, some plain, some have shades of brown. But that characterize the only determinant of the number of spots at the base of the forewing only. I do not know why this species is not named Two Spot, but his name Common Grass Yellow. Perhaps, because this type is the most common.