Why would a bar put a coaster under a beer mug

In the earliest days, the prototype of the coaster was a piece of cloth or paper that covered the cup, and its role was to prevent bugs or other foreign objects from getting into the drink. Later, people found that the role of the cushion on the bottom of the cup is more powerful, so there is a beer mat made of cardboard material.

The functions of the beer mat are as follows:
Separate from water. In the earliest days, bar bars and tables were made of wood, and spilled liquor would seep into the wood, causing moisture, cracking, mold, and decay.
Easy to clean the bar. If the bar and the table are not clean and clean for a long time, the surface will be sticky, which will always affect the overall feeling of the bar. The most important function is to absorb water, when a glass of beer is brought to the front, there is foam and water (condensation on the wall of the cold beer glass), if there is no coaster will find that there will be water at the bottom of the glass, preventing the drinker from raising his glass frequently (imagine, one end of the glass has the wine down the sleeve tube of the forearm). In fact, the role of the coaster is still very big, especially the people who often drink, sit down to drink at a table without a coaster, will feel the whole person is not good.

Non-slip. In later bars, wooden utensils were painted with varnish, and the smooth surface was easy to slide the glass placed on it when it met with water.

Charge it up. In many Western countries, there is a habit of drinking first and paying later. In order to make it easier to remember, the bartender will mark the coaster after the drink is served, usually drawing a vertical line for a glass, followed by four vertical lines and then a horizontal line, just like Chinese write "positive" character to keep count. If there are other drinks, they will be marked with an abbreviation before the mark. In addition, there is another way of accounting, a glass of wine to a coaster, and the final bill is settled in the number of coasters.

Socializing. The bar is a mixed place, and the effect of alcohol is easy to let people down, so the coaster has been developed for social functions. For example, use the back of the coaster to play some small games, write down the "new friend" phone, email, wechat and so on.
In addition, there are small effects such as shock absorption and temperature isolation. Eventually, of course, coasters became part of the bar culture.