Kernig's and Brudzinski's signs
Kernig's and Brudzinski's signs are physical examination results that are strongly suggestive of meningitis. A "stiff neck" is one of the general warning signs of meningitis, and these are some of the first steps to investigate such a finding.
Kernig's sign is essentially a way to demonstrate that the neck is not simply "stiff," but is irritated. With the patient lying flat, the examiner flexes a hip 90 degrees, and then attempts to extend the lower leg at the knee. Pain on extension is a positive sign. If positive but the straight leg raise also produces back pain, the combined sign may be due to low back muscle spasm, herniated disk, or sciatic nerve inflammation.  Clearly, since meningitis can be life-threatening, an ambiguous result here requires further investigation.
To test for the sign, the patient lies on his or her back, and the examiner puts one hand behind the patient’s head and the other on the chest. Using the hand behind the neck to raise the head, but pressing on the chest with the other hand, if the hips and knees flex, the neck sign is positive. 
- Victor, Maurice (2001), Adams and Victor's Principles of Neurology, McGraw-Hill, at 739
- Saberi, Asif & Saeed A. Syed (July 1999), "Meningeal Signs: Kernig’s Sign and Brudzinski’s Sign", Hospital Physician: 23-24
- Goldberg, Stephen (1987), The Four-Minute Neurologic Exam, Medmaster, at 39