The various PDP machines can generally be grouped into families based on word length. With the notable exception of the 16-bit PDP-11, the various architectures all show strong similarities, with the 36-bit PDP-6 and PDP-10 architecture being the most elaborate version.
Members of the PDP series include:PDP-1: The original PDP, an 18-bit machine used in early time-sharing operating system work, and prominent in early hacker culture. One of the first computer games, Spacewar!, was developed for this machine.PDP-2: An unbuilt 24-bit design. PDP-3: First 36-bit machine DEC designed, though DEC did not offer it as a product. The only PDP-3 was built by a customer (alleged to be an intelligence agency) in 1960. Architecturally it was essentially a PDP-1 stretched to 36-bit word width.PDP-4: Supposed to be a slower, cheaper alternative to the PDP-1, but not commercially successful; all later 18-bit PDP machines were based on its instruction set.PDP-5: DEC's first 12-bit machine. Introduced the instruction set later used in the PDP-8.PDP-6: 36-bit timesharing machine. Very elegant architecture. It was considered a large minicomputer or a mainframe.PDP-7: Replacement for the PDP-4; DEC's first wire-wrapped machine. The first version of Unix was for this machine.PDP-8: 12-bit machine with a tiny instruction set; DEC's first major commercial success. Many were purchased by schools, university departments, and research laboratories. Later models were also used in the DECmate word processor and the VT-78 workstation. It is reported that Edson de Castro, who had been a key member of the design team, left to form Data General when his design for a 16-bit successor to the PDP-8 was rejected in favour of the PDP-11; the "PDP-X" did NOT resemble the Data General Nova, although that is a common myth.LINC-8: A hybrid of the LINC and PDP-8 computers; two instruction sets. Progenitor of the PDP-12.PDP-9: Successor to the PDP-7, DEC's first micro-programmed machine.PDP-10: 36-bit timesharing machine, and fairly successful over several different models. The instruction set was a slightly elaborated form of that of the PDP-6.PDP-11: The archetypal minicomputer; a 16-bit machine and another commercial success for DEC. (Also the LSI-11, primarily for embedded systems). The 32-bit VAX series was descended from it, and early VAX models had a PDP-11 compatibility mode. The 16-bit PDP-11 instruction set has been very influential, with processors ranging from the Motorola 68000 to the Renesas H8 and Texas Instruments MSP430, inspired by its highly orthogonal, general-register oriented instruction set and rich addressing modes. The PDP-11 family was extremely long-lived, spanning 20 years and many different implementations and technologies.PDP-12: Descendant of the LINC-8. See LINC and PDP-12 User ManualPDP-13: Designation was not used, apparently due to superstition.PDP-14: A 1-bit machine intended as an industrial controller (PLC). Latter versions (for example, the PDP-14/30) were based on PDP-8 physical packaging technology. I/O was line voltage.PDP-15: DEC's final 18-bit machine. Their only 18-bit machine constructed from TTL integrated circuits rather than discrete transistors. Later versions of the system were referred to as the "XVM" family.PDP-16: A "roll-your-own" sort of computer using Register Transfer Modules, mainly intended for industrial control systems with more capability than the PDP-14. The PDP-16/M was introduced as a standard version of the PDP-16.
Various sites list documents by Charles Lasner, the creator of the alt.sys.pdp8 discussion group, and related documents by various members of the alt.sys.pdp8 readership with even more authoritative information about the various models, especially detailed focus upon the various members of the PDP-8 "family" of computers both made and not made by DEC.