The understand of dimension

Today I used the torch.stack() and confuse of the parameter ‘dim’, I have two tensors of 2*3, why the dim range is [-3,2], and when I set the dim is 2, the results as follow:%E5%BE%AE%E4%BF%A1%E6%88%AA%E5%9B%BE_20180424112159
Thanks a lot