Devices, systems and methods for digital video coding, which includes methods for transform design, are described. In a representative aspect, a method of video processing includes performing a conversion between a current video block of a video and a bitstream of the video based on a rule, wherein the rule specifies that a selection of a transform matrix set for performing a transform operation during the conversion is based on a low-frequency non-separable transform index indicated in the bitstream, wherein the rule specifies that the transform operation includes, during an encoding operation, coding the current video block into the bitstream by applying a forward transform on residual values of the current video block, or wherein the rule specifies that the transform operation includes, during a decoding operation, generating from the bitstream the current video block by applying an inverse transform to scaled coefficients indicated in the bitstream.