Facebook, Microsoft, Twitter and Google's YouTube are coming together to help curb the spread of terrorist content online.

Starting today, the companies commit to the creation of a shared industry database of "hashes" - unique digital "fingerprints" - for violent terrorist imagery or terrorist recruitment videos or images that they have removed from their services. By sharing this information with each other, the companies may use the shared hashes to help identify potential terrorist content on their respective hosted consumer platforms.

The companies will begin sharing hashes of the most extreme and egregious terrorist images and videos they have removed from their services - content most likely to violate all of our respective companies' content policies. Participating companies can add hashes of terrorist images or videos that are identified on one of our platforms to the database. Other participating companies can then use those hashes to identify such content on their services, review against their respective policies and definitions, and remove matching content as appropriate.

In addittion, each company will independently determine what image and video hashes to contribute to the shared database. The companies said that no personally identifiable information will be shared, and matching content will not be automatically removed. Each company will continue to apply its own policies and definitions of terrorist content when deciding whether to remove content when a match to a shared hash is found. And each company will continue to apply its practice of transparency and review for any government requests, as well as retain its own appeal process for removal decisions and grievances.