ten tiers; Tier I is the lowest and Tier X is the highest. With a few exceptions, each nation has a tech tree; smaller navies with one branch and larger navies with branches for each type and a ship of each type at most tiers. In some cases branches split, such as into heavy and light cruisers. The lowest tier for each tech tree ship type varies; cruisers at Tier I, destroyers at Tier II, battleships at Tier III, and aircraft carriers at Tier IV. There are also premium ships that are outside the tech tree; these are available for cash and/or in-game resources. Tier I is essentially a tutorial mode; only one ship of each navy is available in the game at that tier. These ships are classed as cruisers in the game, but historically some were classed as gunboats, avisos, or frigates, and all are from the WWII era. They feature a small number of 102mm to 152mm guns and relatively slow speed. Tier I is unique in that no other tiers are included in their matchmaking, and none of them have torpedoes. Tiers II through X include ships of increasing combat capability, generally following the chronological development of each ship type in each navy, beginning circa 1905–1910 and ending in the 1950s. Ships that were never completed or begun, also called "paper ships", are used to fill gaps in the tech tree and explore hypothetical developments. Most Tier X ships, most Russian and French ships, and all of the German aircraft carriers (Graf Zeppelin reached a late stage of construction but never entered service) are in this category. Players can progress through the game via the research and purchase of ships from each tier. Each specific ship has a number of modules that can be accessed through experience. This experience is used to unlock modules, which can then be purchased with credits and mounted. Once a ship's modules are completely researched (credit purchase is not required), the player can acquire the next ship in the tree by spending experience and credits. The previous ship, if fully upgraded, gains "Elite" status, meaning most of its subsequently earned experience can be converted to free experience by spending doubloons (which are primarily acquired via cash purchases). On any ship, warship elements such as commanders with skill trees and unique perks can be customized, as well as modification kits and consumables