Harmony is the most important principle of design. It refers to the unity achieved through consistent selection and arrangement of objects and ideas. There are five aspects of harmony: shape, size, texture, idea, and color. Lines can create harmony through repetition, contradiction, or transition. Related color harmonies use similar hues, while contrasting harmonies use complementary colors. Sizes, textures, and ideas must also be consistent to achieve harmony in a design. Tests of harmony include whether elements have qualities in common and if decorations suit the purpose and emphasize rather than distract from the design.