[The reflex hammer].

When Erb began to study the knee jerk around 1870 he was able to use the percussion hammer for this purpose. Whereas percussion nowadays is performed with bare fingers, every physician carries a reflex hammer in his bag. Many varieties of reflex hammer have been developed. Their distribution today depends on their use by different schools. The origins and development of the most popular contemporary reflex hammers in English, French and German speaking countries are described.