A video processing method includes determining, for a conversion between a block of a video and a bitstream representation of the video, that a secondary transform with a reduced dimension dimension (e.g., an inverse low frequency non-separable transform) is applicable to a single sub-block of the block in case a dimension of the block satisfies a condition. The secondary transform is performed between a forward primary transform and a quantization step or between a de-quantization step and an inverse primary transform. The reduced dimension is reduced from a dimension of the block. The method also includes performing the conversion based on the determining.