The vest onah beinonit is the 30 days from the start of the last period. This vest is a standard vest for all women, since, as explained earlier, we assume that she may start a regular and exact cycle at any time. Therefore, there is a vest of 30 days, for that was the average cycle in the times of the Gemara. The vest onah beinonit is on the same onah that the most recent period started on. One counts the 30 days of onah beinonit the same way one counts the days of haflagah. Meaning, the 30 days start from the first day of the most recent period, and the last day of the 30 days is the vest.
A woman who has three periods in a row, each one more than 30 days apart, need not keep the vest onah beinonit. Even though the woman did not establish a vest kavua for a specific number of days, she did establish that she never gets a period less than 30 days apart. Therefore, she no longer keeps the vest onah beinonit.
It is common for the vest beinonit and the vest chodesh to be on the same day.