Websites
All these website below are Secondary sources because no one alive right now was alive back in the Medieval period -
http://www.darkknightarmoury.com/c-21-double-headed-battle-axes.aspx
https://www.quora.com/How-common-and-effective-were-battle-axes-in-historical-combat
http://www.lordsandladies.org/battle-axes.htm
http://www.medievalcollectibles.com/c-21-double-headed-battle-axes.aspx
http://www.medievalmiddleages.com/weapons/battle-axe.html
http://www.gransforsbruk.com/en/axe-knowledge/the-history-of-the-axe/
http://www.darkknightarmoury.com/c-21-double-headed-battle-axes.aspx
https://www.quora.com/How-common-and-effective-were-battle-axes-in-historical-combat
http://www.lordsandladies.org/battle-axes.htm
http://www.medievalcollectibles.com/c-21-double-headed-battle-axes.aspx
http://www.medievalmiddleages.com/weapons/battle-axe.html
http://www.gransforsbruk.com/en/axe-knowledge/the-history-of-the-axe/