Just for "fun" (and a little "Oh yeah...? I've got one better then you!" one-upmanship), describe the first system that you used for "CAD" (note that the first computer that you used will be the topic of a similar future question in this forum). For me, it was:
Synercom Informap, an AM/FM/GIS/LIS system running on a PDP-11/70 using the operating system RSX-11M-Plus (anyone remember PIP?). Program customization was done using FORTRAN IV Plus. The workstations were the gigantic, twin-CRT, LSI monsters with 48"x36" digitizer tables. And the plotter was a Calcomp 960 (with a two-pen block).
Yeah, we've come a long way since then!
Intergraph pdp11 , followed by DEC Vax Dec 1980 .... was a terrible upheaval for pen & ink draftsmen ... many never made the transition. Worked happily with Microstation until Oct 2017 .... retirement. Brisbane Australia .