Answer: sliderules

AnswerCrossword Clue
SLIDERULESObsolete calculators
SLIDERULESPlayground protocol?
SLIDERULESEngineers' gear, once
SLIDERULESCalculator predecessors
SLIDERULESEarly calculators
SLIDERULESInstruments used by mathematicians.
SLIDERULES"No pushing" and "Go down feet first"?
SLIDERULESForerunners of pocket calculators
SLIDERULES"Feet first" and "No pushing"?