After an overview of the problems concerning the early rain-gauges and their thresholds, a study is made to investigate the impact that the instrumental threshold of a rain-gauge has on the distribution of precipitation frequency and amount. Some tests have been performed using two historic datasets, i.e. the observations by Giovanni Poleni in Padua from 1725 to 1760, and Jacopo Bartolomeo Beccari in Bologna from 1723 to 1765, and two modern rain-gauge records, i.e. taken at the Botanical Garden, Padua, and at the Hydrographic, Bologna, from 1990 to 2019. The tests consisted in applying a filter to the datasets to simulate the action of an instrumental threshold. The result is that the threshold has an enormous impact on the frequency, and a smaller one on the total amount. The study included how the threshold affects the percentile distribution of precipitation amounts. The results provide indications to correct and interpret early records, and to test their quality. Moreover, they are useful in the analysis of long time series composed by datasets derived from different instruments for climate studies.