That level, however, varies from one system to another. Nearly all digital circuits use a consistent logic level for all internal signals. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that the circuit behaves predictably. Some logic devices incorporate Schmitt trigger inputs, whose behavior is much better defined in the threshold region and have increased resilience to small variations in the input voltage. However, few logic circuits can detect such a condition, and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic level transition. It is usual to allow some tolerance in the voltage levels used for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. Intermediate levels are undefined, resulting in highly implementation-specific circuit behavior. When above the high threshold, the signal is "high". When below the low threshold, the signal is "low". High and low thresholds are specified for each logic family. The two logical states are usually represented by two different voltages, but two different currents are used in some logic signaling, like digital current loop interface and current-mode logic. Logic voltage levels Logic supply voltages For example, it is common to have a read/write line designated R/ W, indicating that the signal is high in case of a read and low in case of a write. Some signals have a meaning in both states and notation may indicate such. Examples of this are the I☬ bus and the Controller Area Network (CAN),and the PCI Local Bus. It also allows for wired-OR logic if the logic gates are open-collector/ open-drain with a pull-up resistor. Logic families such as TTL can sink more current than they can source, so fanout and noise immunity increase. Many control signals in electronics are active-low signals (usually reset lines, chip-select lines and so on). a lower-case n prefix or suffix (nQ or Q_n).For example, the name Q, read "Q bar" or "Q not", represents an active-low signal. The name of an active-low signal is historically written with a bar above it to distinguish it from an active-high signal. Occasionally a logic design is simplified by inverting the choice of active level (see De Morgan's laws). Active-high and active-low states can be mixed at will: for example, a read only memory integrated circuit may have a chip-select signal that is active-low, but the data and address bits are conventionally active-high. The two options are active high and active low. The use of either the higher or the lower voltage level to represent either logic state is arbitrary. Signals with one of these two levels can be used in boolean algebra for digital circuit design or analysis. In binary logic the two levels are logical high and logical low, which generally correspond to binary numbers 1 and 0 respectively.