Similar points can be made about reciprocity (any speaker/sender of a linguistic signal can also be a listener); specialization (linguistic signals do not normally serve any other type of purpose, such as breathing or feeding); non-directionality (linguistic signals can be picked up by anyone within hearing, even unseen); and rapid fade (linguistic signals are produced and disappear quickly). Most of these are properties of the spoken language, but not of the written language. They are also not present in many animal communication systems which characteristically use the visual mode or involve frequent repetition of the same signal. Such properties are best treated as ways of describing human language, but not as a means of distinguishing it from other systems of communication.