A method for decoding video data, the method comprising: inverse quantizing a first coefficient block for a transform unit of a first video block of the video data to generate an inverse quantized first coefficient block for the transform unit of the first video block; applying a first inverse transform to at least part of the inverse quantized first coefficient block for the transform unit of the first video block to generate a second coefficient block for the transform unit of the first video block, the first inverse transform being a non-separable transform signaled by a Non-Separable Secondary Transform, NSST, index for the transform unit of the first video block; generating a residual video block for the first video block, wherein generating the residual video block for the first video block comprises applying a second inverse transform to the second coefficient block for the transform unit of the first video block, the second inverse transform converting the second coefficient block for the first video block from a frequency domain to a pixel domain; forming a decoded video block for the first video block, wherein forming the decoded video block for the first video block comprises summing the residual video block for the first video block with one or more predictive blocks for the first video block; inverse quantizing a first coefficient block for a transform unit of a second video block of the video data to generate an inverse quantized second coefficient block for the transform unit of the second video block; based on the transform unit of the second video block being encoded in transform skip mode, determining that signalling of an NSST index for the transform unit of the second video block is skipped and that no first inverse transform is applied for the transform unit of the second video block; generating a residual block for the second video block, wherein generating the residual video block for the second video block comprises applying the second inverse transform to the inverse quantized second coefficient block for the transform unit of the second video block; and forming a decoded video block for the second video block, wherein forming the decoded video block for the second video block comprises summing the residual video block for the second video block with one or more predictive blocks for the second video block. inverse quantizing a first coefficient block for a transform unit of a first video block of the video data to generate an inverse quantized first coefficient block for the transform unit of the first video block; applying a first inverse transform to at least part of the inverse quantized first coefficient block for the transform unit of the first video block to generate a second coefficient block for the transform unit of the first video block, the first inverse transform being a non-separable transform signaled by a Non-Separable Secondary Transform, NSST, index for the transform unit of the first video block; generating a residual video block for the first video block, wherein generating the residual video block for the first video block comprises applying a second inverse transform to the second coefficient block for the transform unit of the first video block, the second inverse transform converting the second coefficient block for the first video block from a frequency domain to a pixel domain; forming a decoded video block for the first video block, wherein forming the decoded video block for the first video block comprises summing the residual video block for the first video block with one or more predictive blocks for the first video block; inverse quantizing a first coefficient block for a transform unit of a second video block of the video data to generate an inverse quantized second coefficient block for the transform unit of the second video block; based on the transform unit of the second video block being encoded in transform skip mode, determining that signalling of an NSST index for the transform unit of the second video block is skipped and that no first inverse transform is applied for the transform unit of the second video block; generating a residual block for the second video block, wherein generating the residual video block for the second video block comprises applying the second inverse transform to the inverse quantized second coefficient block for the transform unit of the second video block; and forming a decoded video block for the second video block, wherein forming the decoded video block for the second video block comprises summing the residual video block for the second video block with one or more predictive blocks for the second video block.