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Proprietary software, also known as closed source software, is computer software that must be purchased from the creator or owner of the copyright of the source code. Vendors who develop and sell proprietary software are protected by copyright, patents, and other legal means that safeguard their intellectual property. They specify the terms of use in end-user license agreements (EULAs) which govern the way the software can be used and shared.

Proprietary software is sometimes designed to be incompatible with the software from other vendors and may not work on all brands of hardware. This can result in users forming a dependence on the vendor for future versions of the product, and other related products, which is called vendor lock-in or proprietary lock-in. When users are unable to switch to another vendor without significant costs, it can lead to antitrust action.

When software is made available with fewer restrictions on licensing or the source code is made freely accessible, it is called open source software or free software. Open source programs are covered by different kinds of licenses known as copyleft, which enforce free, open access to the source code and its derivatives.