There are two modes for dual-layer orientation, "parallel track path" (PTP) and "opposite track path" (OTP). In PTP mode, used for DVD-ROM, both layers start recording at the inside diameter (ID) with the lead-in and end at the outside diameter (OD) with the lead-out. Sectors are sequenced from the beginning of the first layer to the end of the first layer, then the beginning of the second layer to the end of the second layer. In OTP mode, the second layer is read from the outside of the disk.