In the past, open houses were sleepy, unhurried Sunday afternoon affairs, offering brokers and their clients plenty of time to explore for-sale apartments. Often, they’d have the place to themselves. During the course of the sales process, it was not uncommon for the listing broker to host three or four separate open houses.

But that’s changed dramatically in the face of the city’s inventory shortage. There were 4,795 available Manhattan homes on the market in the second quarter, according to a report by Miller Samuel Real Estate Appraisers, down 31.3 percent from 6,981 in the same quarter of last year. During the same time period, the number of sales jumped 18.8 percent to 3,144, the most second-quarter sales in the past six years.

With listings moving so quickly, brokers said, open houses are often packed with potential buyers, many of whom have been looking for months and have been outbid on other homes.

Although open houses are generally calmer during the summer than in the spring and fall, they now regularly attract up to 40 potential buyers if the home is priced correctly.

These situations can frequently turn into bidding wars, although negotiations usually start after the open house has ended.

If you are a buyer it is crucial to get in the apartment as soon as possible. Many request to see it before the first open house, which is ok if the broker agrees to show it. Otherwise be there at the first available opportunity.