A combination of a lock and a key includes a cylinder having a housing, multiple index rings and multiple cores. The housing has slot laterally defined through the wall thereof the housing. At least one first post and at least one second post respectively extend radially from the first end and the second end of the housing. The index rings are located in the housing and each have at least one column with a first mark. At least one cap restricts the index rings to the housing. Each index ring has one core received therein. The key has a handle, a shaft and multiple unlocking members mounted to the shaft. Each unlocking members has a second mark and a top portion. The index rings are removed and re-arranged by removing the cap via the slot so as to obtain different combinations by the first and second marks.