A method of video processing is described. The method includes performing a conversion between a chroma block of a video region of a video picture of a video and a coded representation of the video according to a rule; wherein the rule specifies that, due to the chroma block having a size M×N, the chroma block is disallowed to be represented in the coded representation using an intra coding mode, wherein M and N are integers that indicate a width and a height of the chroma block respectively, and wherein the intra coding mode includes coding the chroma block based on a previously coded video region of the video picture.