The word wavelength has the figurative usage with allusion to radio reception, implying (mutual) understanding especially in the idiomatic phrase to be on the same wavelength (as someone else).
What idiom was used before "to be on the same wavelength"?
OED's earliest citation for the figurative usage of wavelength is from 1927:
Have one's wave length, know one's sentiments.
American speech (American Dialect Society)
Another idiom that comes to mind is to be on the same page but interestingly it has a later original date than to be on the same wavelength in OED:
‘He..finally told me what page he was on’. ‘Is it th' same page you thought it was?’]
1965 H. Rhodes Chosen Few 179
OED gives a cross reference to the following idiom from the idiom to be on the same page:
to sing the same song (also tune) and variants: (of a number of people) to express the same view, to say the same thing; (also, esp. in to sing from the same hymn (also song) sheet) to present a united front, esp. by being seen publicly to agree. Freq. in political contexts.
The idiom singing the same song predates both idioms mentioned above but the meaning and the usage are a bit different.
Here is OED's earliest citation from 1846:
Priests, greybeards, Braccios, women, boys and spies, All in one tale, each singing the same song, How thou must house, and live at bed and board.
R. Browning Luria iv, in Bells & Pomegranates No. 8. 15/2
Can we say that to sing the same song is the earlier form of to be on the same wavelength? or is there any other idiomatic phrase that was used in the past for the same meaning (perhaps along the same lines of the idiom to be on the same wavelength where there is a figurative usage)?