Although H.261 was first approved as a standard in 1988, the first version was missing some significant elements necessary to make it a complete interoperability specification. Various parts of it were marked as "Under Study". It was later revised in 1990 to add the remaining necessary aspects, and was then revised again in 1993. The 1993 revision added an Annex D entitled "Still image transmission", which provided a backward-compatible way to send still images with 704×576 luma resolution and 352×288 chroma resolution by using a staggered 2:1 subsampling horizontally and vertically to separate the picture into four sub-pictures that were sent sequentially.