Differences in global voltage standards (100V–240V) are the underlying reason for variations in plug designs, but the more direct cause lies in historically formed mechanical differences. American plugs have two flat parallel pins, European plugs have two round pins, British plugs have three rectangular pins with a built‑in fuse, and Australian plugs have two flat slanted pins. These differences mean that travelers must carry plug adapters. The main reasons include: the early electrification of different countries progressed independently, without a unified international standard; British plugs, for historical reasons, adopted a large design with a fuse, emphasizing safety; some European countries use thin round‑pin plugs without grounding (Type C), while Germany and France each evolved their own versions with grounding clips or grounding holes. The International Electrotechnical Commission (IEC) once attempted to promote a unified standard plug, IEC 60906‑2, but it was adopted by only a few countries, such as Brazil. Today, many electrical devices use a power adapter with interchangeable plug heads to adapt to multiple markets, but a truly global standard plug remains a distant prospect.