2016, Miguel Lugo, Competitive Dominoes: How to Play Like a Champion - Second Edition (in Енглески), Dog Ear Publishing, →ISBN, strana 4:
For example, these tiles comprise the suit of 4s:
🁿 🂀 🂁 🂂 🂃 🂄 🂅
Notice that one of these tiles, the 🁑, has a 4 in each side: this is called a “double” tile.