Some flashlights or similar devices take multiple cells in series inserted into a tube, but some flat top cells don't make a good connection in series due to a slightly recessed top.  These spacers are the same diameter as the battery and add a temporary button top so if you put them in a tube with the cells they will stay in place. 

We carry them in 18mm, 21mm, and 26mm diameter.  The 18mm ones work with 18mm diameter cells like 18650, 18350, 18500, etc.  The 21mm ones work with 21mm diameter cells like 21700.  The 26mm ones work with 26mm cells like 26650.