In general it is true that the cheapest air tickets are always in economy class and the cost of air ticket rises according to the number of booked tickets in the air plane. Simply it can be said, that the longer time to departure, the higher probability of finding lower cost. But this is usually not true for very attractive destinations (South America, Caribbean, South-East Asia, Australia) during the most lucrative periods (Christmas and New Year, summer holiday season), when the flights are highly booked even 10 months in advance. Technically it is possible to book air tickets approximately 11 months in advance. Generally it is also true that an air ticket can not be refundable after it is issued and it can be changeable only upon payment (usually unchangeable at all). For searching the lowest cost we recommend you to obey the following rules: