# defaultBehavior The way arithmetic methods round off and handle error conditions. ``` class var defaultBehavior: any NSDecimalNumberBehaviors { get set } ``` ## Discussion By default, the arithmetic methods use the `NSRoundPlain` behavior; that is, the methods round to the closest possible return value. The methods assume your need for precision does not exceed 38 significant digits and raise exceptions when they try to divide by 0 or produce a number too big or too small to be represented. If this default behavior doesn’t suit your application, you should use methods that let you specify the behavior, like [`adding(_:withBehavior:)`](/documentation/Foundation/NSDecimalNumber/adding(_:withBehavior:)). If you find yourself using a particular behavior consistently, you can specify a different default behavior with `setDefaultBehavior(_:)`. The default behavior is maintained separately for each thread in your app. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)