4.241 If I use two signs with one and the same meaning [Bedeutung] then I express this by putting between them the sign “=”.

Thus “a = b” means that the sign “a” is replaceable by the sign “b”.

(If I introduce a new sign “b” by an equation, in which I stipulate that it should replace an already known sign “a”, then I write the equation – the definition – (like Russell) in the form: “a = b Def.”. The definition is a rule for signs.)

OK. Just stipulations here. Ogden has ‘symbolic rule,’ P&McG have ‘rule dealing with signs,’ and Black (p. 211) suggests ‘rule about signs.’