the left grid and right grid must have the same number of elements in all dimensions except the one specified, which will be the sum of the two corresponding 1 <-- 2 <-- creation argument is the index of the dimension by which the join will occur. for n-dimensional grids, the dimensions are numbered from 0 to n-1. in addition, negative numbers from -n to -1 may be used, to which n will be added. Which_dim is the number of the dimension by which the join will occur. For N-dimensional grids, the dimensions are numbered from 0 to N-1. In addition, negative numbers from -N to -1 may be used, to which N will be added. The left grid and right grid must have the same number of elements in all dimensions except the one specified. The result will have the same number of elements in all dimensions except the one specified, which will be the sum of the two corresponding one. For example, joining a RGB picture Dim[y, x, 3] and a greyscale picture Dim[y, x, 1] on dimension 2 (or -1) could make a RGBA picture Dim[y, x, 4] in which the greyscale image becomes the opacity channel. grid to be appended to the left-inlet grid default is 0, which is quite dumb, as Dim() grids can't be joined. you should provide a grid on your own, usually in the right inlet.