A bistable multivibrator implemented as a comparator with hysteresis. Alone, the trigger is a bistable multivibrator. Like every latch, the fundamental collector-base coupled bistable circuit possesses a hysteresis. Such a bistable circuit is the basis of one bit of electronic memory. Many ballpoint and rollerball retractable pens employ this type of bistable mechanism. No timing capacitors are required in a bistable configuration. An example of a mechanical device which is bistable is a light switch. This bistable, hysteretic switch is physiologically necessary for at least three reasons. At the lowest level, bits are stored in a bistable device such as a flip-flop. In bistable mode, the 555 timer acts as a SR flip-flop. This drawing may be variously described as an "ambiguous figure", "reversible figure" or "bistable figure". Static flat-panel displays rely on materials whose color states are bistable. Future research will look at similarities between the two sets of bistable neurons. Chains of bistable flip-flops provide more predictable division, at the cost of more active elements. Some oscilloscopes used a strictly binary (on/off) form of storage known as "bistable storage".