title: ReactJS: Material ui breakpoints
body: <p>What is the difference between <code>breakpoints.up</code>, <code>breakpoints.down</code>,  <code>breakpoints.between</code> and <code>breakpoints.value</code> ?
  And what is meant by this code, how breakpoints value is working here? Is <code>theme.spacing.unit*2*2 = theme.spacing.unit*4</code>  or it has some other meaning?</p>
  
  <pre><code>[theme.breakpoints.up(600 + theme.spacing.unit * 2 * 2)]: {
    width: 600,
    marginLeft: 'auto',
    marginRight: 'auto',
  },
  </code></pre>