title: Merging 2 dataframes
body: <p>I am trying to merge the following 2 dataframes.</p>
  
  <p>df.head():</p>
  
  <pre><code>        x     y     z   w
  0  0.0056    11   824  51
  1  0.0056   138   546  16
  2  0.0056   328  1264  40
  3  0.0056  1212   553  91
  4  0.0056  1839   388  48
  </code></pre>
  
  <p>df1.head():</p>
  
  <pre><code>           x      y     z
  0  5539.0567  12243    27
  1  5873.2923  14474  1540
  2  3975.9776  11353   699
  3  1508.5975   8250   628
  4    66.7913  11812   538
  </code></pre>
  
  <p>using the following command:</p>
  
  <pre><code>df1 = df1.merge(df, how='left',left_on=(['x','y','z']),right_index=True)
  </code></pre>
  
  <p>and the following error crops up:</p>
  
  <blockquote>
    <p>ValueError: len(left_on) must equal the number of levels in the index of "right"</p>
  </blockquote>
  
  <p>In total df has 11458060 rows and df1 has 2528243 rows</p>
  
  <p>I don't really know what this means. Can anyone tell me what I might be doing wrong?</p>