Slide rules! Have a pair of Keuffel+Esser log log duplex decitrig models incoming - a 4181-1 (5" scale all-plastic pocket model) and 4081-3 (10" scale plastic over mahogany standard model). My dad taught me how to use one when I was 8 years old, and I'm getting back into it after finding his old Pickett N600-ES when going through some boxes (this is the same slide rule used by the Apollo astronauts on the first 5 moon missions).
K+E 4181-1:
Attachment 226240
K+E 4081-3:
Attachment 226241
Pickett N600-ES:
Attachment 226242