FLEX CONTENT
flex-direction: row; example 
flex-direction: row-reverse; example 
flex-direction: column; example 
flex-direction: column-reverse; example 

flex-wrap: wrap; example 
flex-wrap: nowrap; example 
flex-wrap: wrap-reverse; example 

flex-flow: row wrap; example 

justify-content: center; example 
justify-content: flex-start; example 
justify-content: flex-end; example 
justify-content: space-around; example 
justify-content: space-between; example 

align-items: center; example 
align-items: flex-start;  example 
align-items: flex-end;  example 
align-items: stretch;  example 
align-items: baseline;  example 

align-content: space-between;  example 
align-content: space-around;  example 
align-content: stretch;  example 
align-content: center;  example 

flex-wrap: wrap;  example 
align-content: flex-start;  example 

Total Center 
 display: flex;
justify-content: center;
align-items: center;  example