Body Structure and Directional Terminology
Position and planes
The reference frame is the anatomical position: standing, face forward, palms forward, feet flat. Planes are described relative to it. A sagittal plane divides left from right (a midsagittal plane splits the body into equal halves), a frontal (coronal) plane divides front from back, and a transverse plane divides upper from lower.
Cavities and regions
The dorsal cavity contains the cranial and spinal cavities. The ventral cavity contains the thoracic cavity (pleural, pericardial, mediastinum) and the abdominopelvic cavity, separated by the diaphragm. Clinicians divide the abdomen into four quadrants (RUQ, LUQ, RLQ, LLQ) and into nine regions: right and left hypochondriac, epigastric, right and left lumbar, umbilical, right and left iliac (inguinal), and hypogastric.
Directional pairs
Anterior/ventral vs posterior/dorsal; superior (cephalad) vs inferior (caudad); medial vs lateral; proximal vs distal; superficial vs deep; bilateral vs unilateral. For body position, supine is face up and prone is face down.
Traps
Proximal and distal apply only to the limbs - toward or away from the point of attachment - never to the trunk, where superior/inferior is correct. The shoulder is proximal to the elbow, not the reverse. Prone and supine are frequently swapped. And hypochondriac here names an abdominal region, not a personality trait.
Worked example
A patient with appendicitis reports pain in the right lower quadrant. The appendix lies in the right iliac region, inferior and lateral to the umbilicus and deep to the abdominal wall - all terms chosen from the anatomical position, not from the patient's posture.