Why does MACD use 12 and 26?

MACD uses 12 and 26 because Gerald Appel designed it in the 1970s when markets traded six days a week.

As the working week used to be 6-days, the period settings of (12, 26, 9) represent 2 weeks, 1 month and one and a half week.

Mechanically, the shorter 12-day moving average is faster and responsible for most MACD movements, while the longer 26-day moving average is slower and less reactive.

The numbers stuck because it is always better to stick to the period settings used by the majority of traders, since buying and selling decisions based on the standards push prices in that direction