Tether is a JavaScript library for positioning UI elements relative to other elements, ensuring they remain attached during scroll, resize, and other interactions. It solves common UI positioning challenges for tooltips, dropdowns, popovers, and similar components through a modular architecture. The core Tether class utilizes plugins for constraint handling, abutment detection, and position shifting, built upon an evented base class for custom event management. The library demonstrates strong engineering practices with clear separation of concerns, comprehensive unit and integration tests (Jest and Cypress), and support for both UMD and ESM module formats. While the positioning logic inherently addresses complex browser edge cases, the codebase is well-structured and maintainable, evidenced by recent commits and a defined release process. Examples and documentation showcase real-world usage patterns. Two reusable components stand out: the evented utility (src/js/evented.js) implements a lightweight event emitter with on, once, off, and trigger methods, demonstrating the observer pattern for decoupled communication. The deferred utility (src/js/utils/deferred.js) provides a promise-like mechanism for asynchronous control flow with resolve, reject, and promise methods. Both modules are minimally coupled, depending only on a type-check utility, and embody proven patterns suitable for extraction into other projects.