I've lived in/near Denver my entire long life. I've always bought windows from Home Depot, Lowe's, whatever, and no one has ever told me I needed high altitude windows...nor have I had any problem with those 'regular' windows in houses, additions, two offices, a triplex, etc, in which I have installed them. I bought these online and installed them in Colorado. All was fine until the next morning when one of them had multiple cracks in a pane. I called the customer service number of the company that supplies and ships them for Amazon, and "Chris" said I needed high altitude windows for anything being installed over 5000 ft altitude. Not one single place on Amazon's website does it say that customers should not buy these if the windows are being installed at 5000 ft or above altitude. That means you should not buy these if you live anywhere along the front range of Colorado, from Trinidad, Colorado Springs, Denver, or Ft. Collins, and of course all the way across the state to Grand Junction. Don't buy them if you live in or near Flagstaff, Arizona; Cheyenne, Wy; Laramie, WY, or Jackson Hole. Los Angeles begins at sea level, but the outskirts run up the mountains to over 5000 ft, so these are not for you. Don't buy them if you live in Santa Fe, NM or Albuquerque, NM. There are many places in the western states over 5000 ft: Washington, Idaho, Montana. According to the company providing and shipping these windows, they are not for you even though they don't say so in their product information. In the event of a problem, everybody on the delivery end of this product has this worked out to cause you the most effort possible, so you won't return them. (Take out the drywall, take out the window, box it up, and ship it back to flatland Illinois.) Fortunately I'm using these as interior-to-interior wall transom windows, and I'll just get another pane cut for one side and clear caulk it in.... it doesn't matter for this install it the two panes are not vacuumed sealed. I cannot recommend this window. The product information is not complete, and I wouldn't trust it if you're not using the window for anything other than a basement in Illinois.