Blu-ray Disc Recordable (or BD-R) refers to two direct to disc optical disc recording technologies that can be recorded on to an optical disc with an optical disc recorder. BD-R discs can be written to once, whereas BD-RE (Blu-ray Disc Recordable Erasable) can be erased and re-recorded multiple times. Disc capacities are 25 GB for single-layer discs, 50 GB for double-layer discs, 100 GB for triple-layer, and 128 GB for quadruple-layer (in BD-R only). The minimum speed at which a Blu-ray Disc can be written is 36 megabits (4.5 megabytes) per second.