A bitrate ladder is the set of resolution-bitrate rungs an asset is encoded into so an adaptive-bitrate (ABR) player can switch between them as bandwidth changes; the ladder is the menu and ABR is the switching machinery. For most of streaming history a service ran every title through one fixed ladder, which quietly assumes every video is equally hard to compress; per-title encoding replaces that with a ladder built from each asset's measured quality. Three decisions shape a good ladder: set the top rung at the lowest bitrate reaching the quality target (commonly VMAF 93 to 95), space the rungs so neighbours stay close (within about 2 VMAF for invisible switching, 1.5 to 2 times apart in bitrate so a player is not stranded), and pick each rung's resolution from the convex hull. The catch is that the number of rungs is itself per-title, and a ladder with too few rungs forces visible quality jumps while too many multiply encoding and storage cost.
