From an operational perspective, the lower bound corresponds to the deposit rate, which can in theory be kept at an even more negative level because of the cost of storing and shipping physical currency. The key point is that, however defined, the lower bound has been found to be below zero.
2  and kicking” from the 1950s to the late 1980s, since then there has been progressively deep uncertainty as to its empirical relevance, possibly also due to the perceived increasing effectiveness of central banks in stabilising inflation.