However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads. For this reason, some "allowance", or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing "interference" and to expedite hand assembly up to the length of engagement. Such allowances, or "fundamental deviations", as ISO standards call them, are provided for in various degrees in corresponding "classes" of fit for ranges of thread sizes. At one extreme, no allowance is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called "interference fit" may even provide for negative allowances, where the PD of the screw is greater than the PD of the nut by at least the amount of the allowance.