This is a normal PA chest xray. When the image is taken, the scapula is protracted to minimize its overlap with the chest wall. Identify the clavicle and the scapula. This line is drawn over one rib. Notice that the posterior part of a particular rib is more horizontal, while the anterior part of the rib slopes from superior to inferior. Find the first rib on the left side of the patient.
Counting ribs on a CXR is a skill that requires some practice. Radiologists typically find rib 1 and count from there. On the left side of the patient, see if you can find the intersection between the anterior part of the 4th rib and the posterior part of the 7th rib. Click here to see if you got it.