The quartz crystals used for low frequency oscillators are usually from the type tuning fork. These display a bell shaped frequency versus temperature dependency. At room temperature the frequency is nominal. At lower or higher temperatures the frequency drops and thus the kept time will start to lag the real time. To achieve an average accuracy over a certain temperature range, the oscillator must be tuned higher at room temperature. The oscillator can be tuned by selecting the correct values for the oscillator capacitors. The parasitic capacitance of the PCB-layout must also be taken into account.