1. Fixing a Mistake by Punching Every Hole
In telegraphy, information was recorded on paper tape by punching holes. A hole meant one thing; no hole meant another. This simple choice of yes or no represents 1 bit of information [1]. Because you cannot undo a punched hole, early operators needed a special way to fix mistakes in the 7-bit ASCII tape system.
When an operator made an error, they would reverse one punch and type DEL (which is 127, or 1111111 in binary). This command punches every single hole in that column. Since all holes are open, the machine treats that spot as a "ignore this character" instruction [7]. If we write "1" for a hole and "0" for no hole, 1111111 is a row with seven open holes. The logic was simple: if you cannot erase a hole, punch them all to make the character meaningless.
2. Why Five Holes Give Only 32 Possible Patterns
The number of possible patterns grows quickly as you add holes. One hole gives 2 options. Two holes give 4 (2 × 2). Three holes give 8. Five holes give 32 possibilities (2 × 2 × 2 × 2 × 2). Each additional hole doubles the number of choices. Six holes offer 64 patterns, seven offer 128, and eight offer 256.
Early telegraph paper tapes had 5 columns of data holes across their width. Later versions used 6, 7, or 8 columns. The tape width for 5-bit codes was usually about 17 mm (about 0.7 inches), while tapes for 6 bits or more were typically about 25 mm (about 1 inch) wide [8].
With only 32 patterns, the alphabet alone takes up 26 spots. That leaves just 6 remaining combinations. This is not enough to fit all 10 digits, punctuation marks, and signals for controlling machines. So how did engineers make this work? They used clever tricks.
3. Switching Between Letters and Figures to Reuse the Same Holes
The solution was "switching." The 5-bit code, which followed the Baudot system, included two special control signals: one for switching to Letters (LTRS) and one for switching to Figures (FIGS) [3]. The meaning of a hole pattern changed depending on which mode was active just before it.
This is similar to how your smartphone keyboard switches between letters and numbers. However, in teleprinters, the machine remembered the last signal to decide how to read the current row. This code was standardized as International Telegraph Alphabet No. 2 (ITA2) by CCITT in 1932 [3]. By using two different tables of meanings, the 32 patterns could represent more than 32 characters.
This code was invented by French telegraph engineer Emile Baudot, who patented it in 1874. The unit of communication speed, the "baud," is named after him [3]. But deciding which pattern meant which letter required careful planning.
4. Donald Murray's Code: Fewer Holes for Common Letters
In 1901, Donald Murray from New Zealand improved the Baudot code. His key innovation was assigning patterns with fewer holes to the most frequently used letters [4]. By using simple patterns for common letters, the total number of punches decreased, making the machinery last longer.
Murray also built a keyboard perforator. When an operator typed a key, it punched holes in a paper tape. A separate machine then read that tape to send the message. This allowed typists to work at their own pace. Typical typing speeds were 30 to 45 words per minute [4]. Each punch created a small circular scrap of paper called "chad" [8].
At telegraph relay stations, received messages were not retyped by hand. Instead, machines read the incoming tape and punched the same pattern onto a new tape to send it down the next line. It is said that shelves of tapes waiting to be relayed lined up in these stations [5].
5. Sprocket Holes: How the Machine Knew When to Read
Paper tape has a single line of small holes, separate from the data holes. These are called sprocket holes [6]. They align with the position of the data holes.
Sprocket holes engaged with gears to move the tape forward and keep it aligned. In optical readers, which used light to detect holes, these sprocket holes also acted as timing signals. They served as a signal indicating when a data hole had reached the reading position [6]. Importantly, these sprocket holes are not counted as part of the character code. A 5-bit symbol still has only 5 data holes.
6. Parity Bits for Catching Errors and the Move to 7-Bit ASCII
If a hole was missed or an extra one appeared, the machine would read the wrong character. To catch these errors, engineers added a "parity" bit [6]. In even parity, an extra hole is punched if needed so that the total number of holes in a row (data plus parity) is always an even number. If the reader sees an odd number of holes, it knows an error occurred. However, this method only detects errors; it cannot fix them. Also, if two holes are wrong, the count might still be even, so the error goes unnoticed.
ASCII uses 7 bits to represent numbers 0 to 127 (since 27 is 128). The first version was published in 1963 as ASA X3.4-1963 [7]. Some explanations say 7 bits were chosen to save communication costs. Others note that paper tape at the time could record 8 bits per column, so the extra bit was used for parity.
The DEL character mentioned earlier is the pattern where all 7 holes are punched. In NC (Numerical Control) machine tools, programs were stored on this same type of tape. The 64-character EIA-244 standard has fallen out of use, and ASCII became the standard. As a leftover from the paper tape era, some CNC machines still measure program size in "feet" or "meters" [9].
7. Try Hole Patterns Yourself with Paper and a Pencil
You can try this logic with just paper and a pencil. Do not use scissors or hole punches. Draw 5 circles in a row to represent the holes. Fill in some circles to make a pattern. Try writing out different patterns and counting them; you should find there are 32 unique combinations.
Next, add a 6th circle. Make sure the total number of filled (black) circles is always even. This simulates even parity. Ask a friend or family member to change just one black circle to white, or vice versa. If the total count becomes odd, you know something changed. If they change two spots, the total might stay even, showing why simple parity can miss some errors.
You may find real punched paper tapes in museums. Look for the line of small sprocket holes running down the center to see how the timing worked.