he Spiral model combines principles from which two other software development models?

The Spiral model combines principles from the Waterfall model and the Iterative model.

1. Waterfall Model: This model is a sequential design process, often used in software development processes, where progress is seen as flowing steadily downwards (like a waterfall) through the phases of conception, initiation, analysis, design, construction, testing, production/implementation, and maintenance.

2. Iterative Model: This model is a particular implementation of a software development life cycle (SDLC) that focuses on an initial, simplified implementation, which then progressively gains more complexity and a broader feature set until the final system is complete.

In the Spiral model, the software is developed in a series of incremental releases. Early iterations, or spirals, produce prototypes. Later spirals produce increasingly complete versions of the engineered system.