Also called the "quantum cosmic tick", is described in Thief of Time as the shortest interval of time anything in the universe can occur. To the History Monks, it is the time it takes for the universe to be broken down and rebuilt by Time (or the time it takes for "now" to become "then"). The tick itself, however, cannot be measured since it actually occurs outside the perception of those within the universe. Attempting to measure the tick results in a paradox that is equated to "opening the box with the crowbar you will find inside" (perhaps referring to a Spike Milligan line): the instrument measuring the tick is supposed to be broken down and rebuilt during the tick. Planck time would be the Roundworld equivalent.