The “winking owl sign” is classically seen on AP radiographs of the thoracolumbar spine. The eyes of the owl are the pedicles, the head of the owl the rectangle of the vertebral body, and the “beak” the spinous process. You can imagine the spine looking like a stack of owl heads. In the image, the left pedicle of T9 (the left eye of T9) is not visible, suggesting destruction of the cortical bone of this pedicle. This is classically an osteolytic process like metastatic disease. This particular person had prostate cancer.
How do you know it’s T9?: by counting ribs from the bottom.