H.261 was originally designed for transmission over ISDN lines on which data rates are multiples of 64 kbit/s. The coding algorithm was designed to be able to operate at video bit rates between 40 kbit/s and 2 Mbit/s. The standard supports two video frame sizes: CIF (352×288 luma with 176×144 chroma) and QCIF (176×144 with 88×72 chroma) using a sampling scheme. It also has a backward-compatible trick for sending still images with 704×576 luma resolution and 352×288 chroma resolution (which was added in a later revision in 1993).