Question: what is a 2u shelf?
Answer: For electronic equipment racks (e.g., 19 or 23 inches wide), the term 1U is used to define one rack unit of height. To be precise, 1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75”, which equals 3.5-inches high. A 3U height would be 3 x 1.75” = 5.25-inches.