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When you purchase an HD video on a supported device or computer, only HD video will be downloaded. To download the SD version, you need to download the video again from your Purchased page. Conversely, if you purchase an HD video on an unsupported device, the SD version will be downloaded. Then, you will need to download the HD version from your Purchased page. Learn more about downloading previous purchases.
Note: Some previously purchased movies may not be available in iTunes in the Cloud. At the time of purchase, these titles will continue to download both HD and SD version to your Mac or PC. These movies will indicate that they are not available in iTunes in the Cloud on their product details page in the iTunes Store.