Why do some old computers use 7 or 9 bits for a byte?

In one line

The rule

A byte is a group of bits (binary digits) used to store data, but the number of bits in a byte has changed over time and between computers.

Today, a byte is 8 bits, but in the past, it could be 7, 8, or 9 bits, depending on the computer.

Let's understand it simply

Imagine you are packing mangoes in boxes. Today, every box holds 8 mangoes. But long ago, some shops used boxes for 7 or 9 mangoes. Why? Maybe their shelves were smaller, or their customers wanted a different number. Computers are like these shops. Each computer has a memory that stores data in boxes called bytes. The size of these boxes (how many bits) was not always the same. It depended on what the computer needed and how it was built.

How it works

1. Bits (binary digits) are the smallest units of data: 0 or 1.
2. A byte is a group of bits, used to store one character (like a letter or number).
3. Early computers had different hardware designs. Some grouped 7 bits, some 8, some 9, as one byte.
4. The number of bits per byte affected how much data could be stored and processed at once.
5. Over time, 8 bits became the standard, but older systems kept their own sizes for compatibility or technical reasons.

The story behind it

In the 1960s and 1970s, computers were made by many companies, each with its own ideas. IBM, DEC, and others built machines for different uses: business, science, or communication. There was no rule about how many bits a byte should have. For example, IBM’s early computers sometimes used 6 or 7 bits for a character, while DEC’s PDP-10 used 9 bits. The 8-bit byte became standard only after the rise of personal computers in the late 1970s and 1980s, especially with the success of the IBM PC.

Where you see it in real life

• Old telecommunication systems: Used 7 bits to store ASCII characters (letters, numbers, symbols) because ASCII only needed 128 values.
• DEC PDP-10 computers: Used 9 bits per byte, which allowed them to store more information per character or support special symbols.
• IBM mainframes: Some used 6 or 7 bits per byte for early business data processing.
• Punch cards: Early data storage cards often used 6 or 7 bits per character, matching the hardware.
• Early printers and terminals: Some only understood 7-bit codes, so computers matched their bytes to this size.
• Modern Unicode text: Even today, you might see 7-bit or 8-bit settings in email or text file formats for compatibility.

Why it matters

• Compatibility: Old software and data files may use 7 or 9 bits, so modern computers must sometimes handle these sizes.
• Data storage: The number of bits per byte affects how much memory is needed for text, numbers, or images.
• Communication: Some networks and devices still use 7 bits for text, especially in simple messages (like SMS).
• Understanding history: Knowing about 7- and 9-bit bytes helps you understand why some old files or systems behave strangely today.

Common mistakes

• Thinking a byte is always 8 bits: Today it is, but not always in the past.
• Confusing bit and byte: A bit is a single 0 or 1; a byte is a group of bits (not always 8).
• Assuming all computers use the same byte size: Different machines had their own standards.
• Believing 7 or 9 bits is a mistake: It was a design choice, not an error.

Remember it

A byte was not always 8 bits—older computers used 7 or 9 bits, depending on their needs and design.