for better tactical mobility and ease of strategic transport. These are intended for armoured reconnaissance, skirmishing, artillery observation, expeditionary warfare and supplementing airborne or naval landings. Light tanks are typically cheaper to build and maintain, but fare poorly against heavier tanks. They may be held in reserve for exploiting any breakthroughs in enemy lines, with the goal of disrupting communications and supply lines. Medium tanks are mid-sized tanks with adequate armour and guns, and fair mobility, allowing for a balance of fighting abilities, mobility, cost-effectiveness, and transportability. Medium tanks are effective in groups when used against enemy tanks. Heavy tanks are larger tanks with thick armour and more powerful guns, but less mobile and more difficult to transport. They were intended to be more than a match for typical enemy medium tanks, easily penetrating their armour while being much less susceptible to their attacks. Heavy tanks cost more to both build and maintain, and their heavy armour proved most effective when deployed in support infantry assaulting entrenched fortifications. Over time, tanks tended to be designed with heavier armour and weapons, increasing the weight of all tanks, so these classifications are relative to the average for the nation's tanks for any given period. An older tank design might be reclassified over time, such as a tank being first deployed as a medium tank, but in later years relegated to light tank roles. Tanks were also classified by roles that were independent of size, such as cavalry tank, cruiser tank, fast tank, infantry tank, "assault" tank, or "breakthrough" tank. Military theorists initially tended to assign tanks to traditional military infantry, cavalry, and artillery roles, but later developed more specialized roles unique to tanks. In modern use, the heavy tank has fallen out of favour, being supplanted by more heavily armed and armoured descendant of the medium tanks – the universal main battle tank. The light tank has, in many armies, lost favour to cheaper, faster, lighter armoured cars; however, light tanks (or similar vehicles with other names) are still in service with a number of forces as reconnaissance vehicles, most notably the Russian Marines with the PT-76, the British Army with the Scimitar, and the Chinese Army with the Type 63. Main battle tank Modern main battle tanks or "universal tanks" incorporate recent advances in automotive, artillery, armour, and electronic technology to combine the best characteristics of the historic medium and heavy