From the late eighteenth century onward, composers began to use dynamic signs as an independent layer of expressive meaning, deployed either to reinforce the dynamic tendencies inherent in the music (through melodic or harmonic trajectory, phrasing, texture, etc.) or to counteract them. The study of this phenomenon thus far has been limited to individual observations regarding particular events, such as "surprise dynamics" (for example sudden peaks in volume or a subito piano following a crescendo), with no attempt to reach a systematic understanding. This article elucidates a little-noticed device, the preventive or cautionary dynamic. Some nineteenth-century composers reiterate dynamic instructions in their instrumental music several times in succession. The purpose of these repetitions is to maintain a desired dynamic level that runs counter to the intrinsic ebb and flow of tonal music (the "internal dynamic" as defined by some authors). The device is used most frequently at the extremes of the dynamic spectrum, often to create expressive tension in a poetic/extramusical sense. The first composer to make substantial use of this technique was Beethoven, but it is Mendelssohn who developed and deployed it most fully. This type of dynamic instruction is most prevalent in large-scale instrumental compositions, such as the symphony, and highlights another aspect of musical practice of the period: composers increasingly had to contend with conductors and performances over which they had no direct control. The increased use of dynamic signs in general and preventive or cautionary dynamics in particular can thus be read as an attempt to assert control over expressive aspects of their compositions that traditionally had been left to performers.