result will instead be dataframe (axis= is the columns)in [ ]pd concat([ ]axis= out[ ] nan nan nan nan nan nan nan nan nan nan nan nan nan nan in this case there is no overlap on the other axiswhich as you can see is the sorted union (the 'outerjoinof the indexes you can instead intersect them by passing join='inner'in [ ] pd concat([ ]in [ ]pd concat([ ]axis= out[ ] nan nan in [ ]pd concat([ ]axis= join='inner'out[ ] you can even specify the axes to be used on the other axes with join_axesin [ ]pd concat([ ]axis= join_axes=[[' '' '' '' ']]out[ ] nan nan nan nan one issue is that the concatenated pieces are not identifiable in the result suppose instead you wanted to create hierarchical index on the concatenation axis to do thisuse the keys argumentin [ ]result pd concat([ ]keys=['one''two''three']in [ ]result out[ ]one two three much more on the unstack function later in [ ]result unstack(out[ ] data wranglingcleantransformmergereshape www it-ebooks info