Why Vdd and Vss not just Vd and Vs?

[NuLED]

I don't understand why they named it with double letters?

I know it was from ages back between Bi-polar Junction Transistors and FETs but why double up the letters?

 

[CluQu]

Consistency I suppose. If it ain't broke, don't fix it.

 

[NuLED]

ok but then why double up the letters in the first place for the collector/emitter?

Vcc and Vee