ITA2 and the American Teletypewriter Code
Pick up a code card for a Teletype Model 28 and look at Figures-F. It says %. Now go to the nearest Model 28 on your bench, shift to FIGS, and press the F key. It prints !.
The card is not wrong. Your machine is not wrong. They are simply using two different national dialects of the same five-unit code.
ITA2 and Its Open Slots
The five-bit code used by all RTTY is ITA2 — International Telegraph Alphabet No. 2, standardized by the CCITT in the 1930s on the foundation Donald Murray had laid around 1905. ITA2 defines 32 unique bit patterns across two shift states (Letters and Figures), but it does not fill every slot. Several figures-shift positions were deliberately left as “reserved for national use” — a formal acknowledgment that different countries would need different punctuation and control characters.
Every national variant of ITA2 sends exactly the same bits for letters A through Z, for digits 0 through 9, and for the shift control codes (LTRS, FIGS, CR, LF, BEL, DEL). The variants differ only in how they assign meaning to those open figures slots. On the wire, two machines running different national variants are exchanging identical bit patterns. What appears on paper depends entirely on how the receiving machine’s type basket was cast.
The Teletypewriter Code: America’s ITA2
The Teletype Corporation — the dominant American manufacturer from the 1920s through the 1980s — standardized its own figures assignment for all its equipment. This is the Teletypewriter Code, commonly abbreviated USTTY (US TeleTYpewriter code) or referred to simply as “the US arrangement” or “the American arrangement.”
The key differences from international ITA2 are all in the figures case. The most visible: ITA2 assigns % to the F-key; USTTY assigns !. The bell character also moves: ITA2 puts BELL on the J-key, USTTY puts it on the S-key, swapping with the apostrophe. The H-key in USTTY carries STOP, a dedicated end-of-message marker used in formatted commercial and military traffic.
| Key (FIGS) | ITA2 (International) | USTTY (American) |
|---|---|---|
| F | % (percent) | ! (exclamation) |
| S | ’ (apostrophe) | BEL (bell) |
| J | BEL (bell) | ’ (apostrophe) |
| H | (national use) | STOP |
These differences are small in number but significant in practice. A machine expecting USTTY and receiving traffic formatted for international ITA2 will ring its bell at unexpected moments (when an apostrophe was intended) and silently miss it at others. An exclamation mark transmitted by a USTTY sender will arrive as a blank space on a true ITA2 receiver. The codes are interoperable for plain text in most cases; the hazards lie in punctuation, control characters, and formatted traffic.
The Type Basket Rules
The practical reality for anyone reading a code card: the five-bit pattern that leaves the sender is fixed. What appears on the paper at the receiving end is determined by the type basket — the physical ring of typebars or the printwheel slug cast for that specific character position. A sender has no way to change what a remote machine prints; that was decided at the factory.
Teletype Corporation cast its American machines with ! in the F-key type slug, bell circuitry wired to the S-key code, and STOP in the H-key position. Every Model 15, 19, 28, 32, 32ASR, and 33 that left the Skokie plant carried the same USTTY figures basket. The code card labeled “ITA2” in the collection is technically accurate — USTTY is a national variant of ITA2 — but it describes the international standard assignment, not the one your machine implements.
The Military Pipeline
The reason virtually every mechanical teletype in amateur hands uses the USTTY arrangement has little to do with the amateur community’s own standards and everything to do with the United States military.
USTTY became the standard teletype code for the US Army, Navy, and Air Force through the middle decades of the twentieth century. Military communications networks ran Teletype Corporation machines on USTTY throughout World War II, Korea, and Vietnam-era logistics traffic. By the 1950s and 1960s, the armed forces were the single largest user of landline and radio teletype equipment in the country.
When those circuits were upgraded or decommissioned, the machines entered the surplus market. Model 15s, 19s, and 28s — robust, well-maintained, and now far cheaper than anything a hobbyist could have bought new — turned up at surplus dealers, military auctions, and hamfests across North America. Amateur radio operators who had been watching the commercial RTTY services with interest suddenly had access to the hardware.
The first serious amateur RTTY activity in North America, in the late 1940s and early 1950s, was built almost entirely on this surplus foundation. The Western Union and Postal Telegraph upgrades of the 1940s had put similar Teletype machines onto the market a decade earlier; former Press Wireless operators like Forrest “Bart” Bartlett W6OWP were at the center of the early amateur community and already knew the equipment intimately from their commercial careers.
The North American amateur standard — 45.45 baud, 170 Hz shift, lower sideband — was effectively inherited from these machines. So was the figures arrangement. In amateur RTTY circles the USTTY assignment is often called the “military arrangement” or simply “the US code,” a colloquial acknowledgment of where the hardware came from. The name stuck even as the amateur community moved to terminal units, software decoders, and sound cards.
Today ITTY transmits on 45.45 baud USTTY, the same code and speed at which those surplus Model 19s first went on the air more than seventy years ago. Owners of restored Teletype machines connect, tune their demodulator, and watch their ! print exactly where the designers of the type basket intended it.
For the technical history of five-unit codes from Baudot through Murray through ITA2, see Alan Hobbs G8GOJ’s authoritative treatment: Five-Unit Codes for the Radio Amateur →
For a first-hand account of early amateur RTTY on the West Coast, including the surplus equipment era: Letter of Recollections — Forrest Bartlett W6OWP →