In theme-and-variations form, the element that is usually not changed is the basic melody or theme. The theme is introduced at the beginning of the composition and remains recognizable throughout the different variations, even as it undergoes changes in rhythm, harmony, texture, and other musical elements. The variations are created by altering different aspects of the theme while still retaining its essential character. This form of composition allows the composer to explore and develop different musical ideas within a cohesive structure, while maintaining a strong sense of unity and coherence.