The behaviour of a monostable trigger circuit, which is triggered with periodic pulse, trains is analysed by determining the sequence of its division ratio. Some general properties of these sequences are found, from which it follows that the sequence may show irregular changes, periodical changes or take a constant value of the division ratio after a sufficient number of cycles. Some conditions are found which must be satisfied for the sequence to have one of the above-mentioned properties. Periodical changes are obtained only in exceptional cases.