Bootstrap 4 grid tiers and media query breakpoints
xs = Extra small <576px
Max container width None (auto)


sm = Small ≥576px
Max container width 540px


md = Medium ≥768px
Max container width 720px


lg = Large ≥992px
Max container width 960px


xl = Extra large ≥1200px
Max container width 1140px


Bootstrap 4 media queries code snippet


Code:

/*
####################################################
M E D I A  Q U E R I E S
####################################################
*/


/*
::::::::::::::::::::::::::::::::::::::::::::::::::::
Bootstrap 4 breakpoints
*/


/* 
Extra small devices (portrait phones, less than 576px) 
No media query since this is the default in Bootstrap because it is "mobile first"
*/




/* Small devices (landscape phones, 576px and up) */
@media (min-width: 576px) {  


}


/* Medium devices (tablets, 768px and up) The navbar toggle appears at this breakpoint */
@media (min-width: 768px) {  


}


/* Large devices (desktops, 992px and up) */
@media (min-width: 992px) { 


}


/* Extra large devices (large desktops, 1200px and up) */
@media (min-width: 1200px) {  
    
}






/*
::::::::::::::::::::::::::::::::::::::::::::::::::::
Custom media queries
*/


/* Set width to make card deck cards 100% width */
@media (max-width: 950px) { 


}