Year  18-bitters     12-bitters         16-bitters                36-bitters

1960    PDP-1 --------------------------------------------------------
1961      |                                                           \
1962    PDP-4 <--- LINC --------                                       \
1963      |        PDP-5   \    \                                       |
1964    PDP-7        |      \    \                                   PDP-6
1965      |        PDP-8 --\ |    \                                     |
1966      |        PDP-8/S LINC-8  |                                    |
1967      |          |       |     |                                 KA10
1968    PDP-9      PDP-8/I,L |     |                                    |
1969      |          |     PDP-12  |                                    |
1970    PDP-15       |           PDP-14  PDP-11(/20)                    |
1971      |        PDP-8/E                /   |  \                      |
1972    PDP-15/76  PDP-8/M        - PDP-11/05 | PDP-11/45 --         KI10
1973                 |           /   |    PDP-11/40  |      \           |
1974                 |          /    |         |     |       \          |
1975               PDP-8/A PDP-11/03 PDP-11/04 |     |    PDP-11/70  KL10
1976                 |            PDP-11/34    | PDP-11/55    |         |
1977               VT78              |    PDP-11/60           |         |
1978                              PDP-11/34C              VAX-11/780 KS10

The first DEC 36-bit system, the PDP-6, came out in 1964.  The PDP-10
line consisted of the KA which arrived in 1967, the KI in 1972.  The
KL came out early in 1974; the KS was a 1978 entry.  In 1984 the
unreleased KC was canceled.

Source: C.G. Bell, et al's "Computer Engineering" 1978 (mostly)

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