Pilots are required to accumulate a certain number and type of flight hours for each certificate or rating.[1][6][7][8] The requirements become more numerous with each successive rating, but most requirements can be "stacked" (i.e. flying cross-country in instrument conditions fulfills both cross-country and instrument hour requirements). Detailed requirements for each rating can be found in 14 CFR Part 61[9] and in the sections to follow.