Which tool is primarily used for creating designs or patterns in hair?

The primary tool used for creating designs or patterns in hair is the trimmer. Trimmers are designed for precision work, allowing barbers to create detailed outlines and sharp designs. They are typically smaller and more maneuverable than other hair cutting tools, enabling the barber to achieve intricate patterns and finely detailed work near the hairline or within the scalp.

While scissors are often utilized for cutting and layering, they aren't the primary choice for design work due to their length and cutting style. Clippers are generally used for bulk hair removal and creating fades but lack the finesse required for design precision. Razors may be used for texture and blending but are not typically the first choice for creating defined patterns. Thus, the trimmer stands out as the best option for this specific task.