It is common that the term Claymore is attributed to the Scottish two handed sword romanticised so often. This was a very large sword and uncommon in Europe as most European swords of this size were mainly ceremonial. It is commonly depicted with the forward angled arms and quarterfoil designs at the end. The classic medieval sword was large and very much the same as we see so often in movies depicting knights especially the crusaders. The quarterfoil is probably made famous from the seal of John Balliol. The 'what is referred to' as the Claymore was most likely a larger version of this sword.
It seems however that Claymore is the wrong term for this formidable weapon. To get to the reason we need to go forward in time to the Jacobites who predominantly used the basket hilted swords. The Scottish version was broader than similar swords of the time. This sword was called a claidheamh-mór. This is Gaelic and translates into Great Sword due to its larger size than its contemporaries. It is well accepted that Claymore is derived from the Gaelic claidheamh-mór.
The Basket hilted sword was also called the Scottish Broad Sword. There was also a version called the claidheamh cuil which means back sword. The back was blunt with just one sharp edge. The long two hander was called a claidheamh dà làimh, translated it literally means two handed sword.In the romanticised period after the Jacobites the term Claymore was then applied to the long medieval sword.
The British army and many other armies had similar hilted swords but the blades were more slend. A common weapon among the clansmen during the Jacobite rebellions of the late 17th and early 18th centuries was the Scottish Basket Hilted Broadsword, commonly known as claidheamh mor or "claymores" – meaning "great sword" in Scottish Gaelic. In close quarters, the claymore was the ideal weapon of choice for combating British soldiers armed with long, unwieldy, muskets with plug bayonets. When paired with a "targe", or light buckler a highlander was provided with a staunch defense, allowing him to block a bayonet with the targe and then deliver a thrust with the sword into his opponent's torso.
At range, this strategy would do little against musket armed troops firing in volleys or artillery using canister shot (while effective against bayonets, the targe would not fare so well against a musket ball), which necessitated tactics such as the "Highland Charge", which required a Jacobite war band to close with their targets as quickly as possible, normally under heavy fire, using the smoke from musket and cannon fire to cover the last leg of the assault before attacking the line. Naturally, even this risky strategy, led to many casualties among the Jacobite clansmen against disciplined volley fire and massed artillery firing grape and canister shot.
In between rebellions, after the disastrous Jacobite defeat at the Battle of Culloden and the overall failure of the rebellions, it would become illegal to carry the claymores and targes and so many Highlanders would hide these weapons in the heath. It is not an uncommon story that features a hiker finding such a blade while walking.
Click Here for more information