Why is sum(X, 1) the sum of the columns in MATLAB?

Question

>> X = [0 1 2
        3 4 5]

>> sum(X, 1)

ans =

     3     5     7

sum(X, 1) should sum along the 1st dimensio

Answer 1

1 means column, according to http://www.mathworks.com/access/helpdesk/help/techdoc/ref/sum.html

B = sum(A,dim) sums along the dimension of A specified by scalar dim. The dim input is an integer value from 1 to N, where N is the number of dimensions in A. Set dim to 1 to compute the sum of each column, 2 to sum rows, etc.