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Reach is an important factor in melee combat that represents the distance a which a fighter can strike its oponent. For instance, spears and halberds have a higher reach than short swords or battle axes. Mounted units also add a reach bonus depending on their height, representing the advantageous position the rider gets from being more elevated above the battlefield.
When 2 melee units are fighting, the Reach difference between the 2 affects the odds of 'touching' the opponent: the unit with the highest reach has an advantage as he can keep its foe at distance and can touch him without being reachable himself.
Reach also gives a bonus against units of higher mass and movement speed, as it can turn their force against them. Spear infantry with high reach will therefore be more effective against heavy mounted units such as cavalry or elephants.