The 12 standard meridians, also called Principal Meridians, are divided into Yin and Yang groups. Qi is said to circulate in the meridians. Qi travels through meridians similar in description to the nadis. Neither "qi" nor meridians exist. Scientific research has not supported the existence of "qi", meridians, or yin and yang. The Yin Meridians of the leg are Spleen, Kidney, and Liver. The Yin meridians of the arm are Lung, Heart, and Pericardium. Great circles through the poles are called lines of longitude (or meridians). A circle of latitude is perpendicular to all meridians. For cylindrical projections, the axes of the ellipse are aligned to the meridians and parallels. A horizontal line passes through both zero and the 180th meridians. A common scheme places them at each intersection of displayed meridians and parallels. This method also breaks down at the 180 meridians and the poles. They have a different focal power in different meridians. There's also a number of less customary channels branching from the "regular" meridians.