When one thinks of filters, one thinks of classical filters in traditional engineering textbooks -- for example lowpass, highpass, bandpass, and bandstop filters. In the field of audio, one encounters other filters as well. In this tutorial we take a deeper look at allpass filters – where phase (instead of frequency shaping) is used as a manipulating tool. We start with the basics of allpass filters and look at how they may be used for audio processing. Then we look at how allpass filters may be used as building blocks for more complicated topologies to create even more interesting filters.