As a non-native English writer, of course, I asked myself why this block is called Periwinkle. I soon found out that periwinkle is the name of an animal (snail) as well as a plant. And I actually do see both of them in this block.
The periwinkle snail’s shell roughly has this shape, so you can “see” four periwinkles in this quilt block.
(I admit, this is not a periwinkle, but a sea snail with a similar shell. I wasn’t able to find a picture of a periwinkle that I could post here.)
The petals of the plant periwinkle have this shape, usually five petals to one blossom, so you can “see” a periwinkle blossom with only four petals in this quilt block.
Never mind what you see, this block with its austere geometry wasn’t too easy to plan and sew, despite its easy looks.