The a:b:c notation for sampling ratios, as found in the ITU-R BT.601 specifications, has the following meaning: a) 4:2:2 means 2:1 horizontal downsampling, no vertical downsampling. Think four Y samples for every two Cb and 2 Cr samples in a scan line. b) 4:1:1 ought to mean 4:1 horizontal downsampling, no vertical. Think four Y samples for every one Cb and one Cr samples in a scan line. It is often misused to mean the same as 4:2:0. c) 4:2:0 means 2:1 horizontal and 2:1 vertical downsampling. Think four Y samples for every Cb and Cr samples in a scan line. Not only is this notation not internally consistent, but it is incapable of being extended to represent any unusual sampling ratios, that is different ratios for the Cb and Cr channels.