The equal sign was first used by Robert Recorde in The Whetstone of Witte (1557) in order “to auoide the tediouse repetition of these woordes : is equalle to : I will sette as I doe often in woorke use, a paire of parralles, or Gemowe lines of one lengthe, thus : ==, bicause noe 2, thynges, can be moare equalle”.
Prior to its present incarnation the equal sign was often expressed with the word aequales that was sometimes abbreviated to just aeq, (which would suggest an interesting ligature design).