Check out example codes for "one-sided box shadow css". It will help you in understanding the concepts better.

Code Example 1

  border: 1px solid #333;
  width: 100px;
  height: 100px;

  /* offset-x | offset-y | blur-radius | spread-radius | color */
  box-shadow: 10px 0 5px -2px #888;

     Position the shadow on the side of the element
     that you want it to be, then add a negative spread radius

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