An intra sub-partition, ISP, method of decoding a video signal, the method comprising: partitioning a block of video data into a plurality of sub-partitions, wherein each sub-partition comprises N×M samples where N and M are number of samples in horizontal or vertical direction; obtaining a number of most probable intra predictions based on intra prediction modes of neighboring blocks of the block and determining an intra prediction mode to predict the samples of the block sub-partitions; selecting an optimal transform combination from DCT-II, DST-VII and DCT-VIII transform kernels for each sub-partition, wherein selecting the optimal transform combination comprises applying a multiple transform selection, MTS, mode to the ISP, wherein the optimal transform combination is indicated by one MTS flag and one MTS index; obtaining residual coefficients of a first sub-partition and deriving dequantized residual coefficients of the first sub-partition by performing inverse-quantization of the residual coefficients of the first sub-partition; deriving residual samples of the first sub-partition by performing inverse transforms on dequantized residual coefficients of the first sub-partition in horizontal and vertical directions, wherein the inverse transforms are based on the optimal transform combination of the first sub-partition; deriving prediction samples of the first sub-partition by performing intra prediction of the first sub-partition based on neighboring reconstructed samples adjacent to the first sub-partition; deriving the reconstructed samples of the first sub-partition by adding the residual samples of the first sub-partition to the prediction samples of the first sub-partition; and deriving prediction samples of a second sub-partition by performing intra prediction of the second sub-partition based on the reconstructed samples of the first sub-partition, characterized in that applying the MTS mode to the ISP comprises: selecting horizontal and vertical transforms by signaling one MTS flag for the block and one MTS index for each sub-partition, wherein the MTS flag is shared by sub-partitions. partitioning a block of video data into a plurality of sub-partitions, wherein each sub-partition comprises N×M samples where N and M are number of samples in horizontal or vertical direction; obtaining a number of most probable intra predictions based on intra prediction modes of neighboring blocks of the block and determining an intra prediction mode to predict the samples of the block sub-partitions; selecting an optimal transform combination from DCT-II, DST-VII and DCT-VIII transform kernels for each sub-partition, wherein selecting the optimal transform combination comprises applying a multiple transform selection, MTS, mode to the ISP, wherein the optimal transform combination is indicated by one MTS flag and one MTS index; obtaining residual coefficients of a first sub-partition and deriving dequantized residual coefficients of the first sub-partition by performing inverse-quantization of the residual coefficients of the first sub-partition; deriving residual samples of the first sub-partition by performing inverse transforms on dequantized residual coefficients of the first sub-partition in horizontal and vertical directions, wherein the inverse transforms are based on the optimal transform combination of the first sub-partition; deriving prediction samples of the first sub-partition by performing intra prediction of the first sub-partition based on neighboring reconstructed samples adjacent to the first sub-partition; deriving the reconstructed samples of the first sub-partition by adding the residual samples of the first sub-partition to the prediction samples of the first sub-partition; and deriving prediction samples of a second sub-partition by performing intra prediction of the second sub-partition based on the reconstructed samples of the first sub-partition, characterized in that applying the MTS mode to the ISP comprises: selecting horizontal and vertical transforms by signaling one MTS flag for the block and one MTS index for each sub-partition, wherein the MTS flag is shared by sub-partitions.