General description Royal Mail Large Letter PIP Cardboard Postal Boxes are high-quality, UK-manufactured corrugated boxes designed for the Royal Mail 'large letter' postal system. These single wall die cut boxes are available in various sizes, including C6, C5, and C4, making them ideal for sending documents and small items securely. Technical specifications C6 Size - Internal: 160 x 110 x 20 mm C6 Size - External: 166 x 114 x 22 mm C5 Size - Internal: 222 x 160 x 20 mm C5 Size - External: 228 x 164 x 22 mm C4 Size - Internal: 332 x 245 x 22 mm C4 Size - External: 332 x 245 x 24 mm Installation These boxes are easy to assemble with fold-over ends and "earlock" lids, allowing for quick and efficient packing.