Units of information: A bit is either a 0 or a 1.

The idea

A bit is the smallest unit of data a computer stores, and it has exactly two possible states: 0 or 1. Physically these states are represented by two distinguishable levels, such as a low or high voltage, or a switch that is off or on. Because there are only two values, a single bit cannot represent a whole number like 5 or a letter like 'A' on its own; it can only record one binary choice. To store richer information, bits are grouped: 8 bits make a byte, and a byte can hold 256 different patterns (2⁸), enough for a character code. When you answer a question about a bit, check that you are describing one binary digit with two possible values, not a byte or a group of bits.

What earns the marks

• A bit has exactly two possible values: 0 or 1.
• A bit is a single binary digit, the smallest unit of data.
• The two states can be represented physically, for example by two voltage levels or a switch being off or on.
• A single bit alone cannot represent a full character or a large number; bits are grouped (for example 8 bits = 1 byte) to represent more information.

Common mistakes and how to put them right

• Saying a bit can be any number from 0 to 9.

  Put it right: A bit is binary, so it is only 0 or 1.

• Confusing a bit with a byte.

  Put it right: A byte is a group of 8 bits, whereas a bit is a single binary digit.

• Claiming a bit can store a letter such as 'A' by itself.

  Put it right: One bit has only two states, so characters need a group of bits.

Exam tips

• Read the question carefully to see whether it asks about a bit or a byte before choosing an answer.
• Remember the two values are 0 and 1, not 'true/false' wording unless the question uses it.
• If an option describes a group of 8 binary digits, that is a byte, not a bit.