how to apply multiple classes in css code example

Example 1: css multiple classes same rule

.bucket1, .bucket2, .bucket3 {
}

.bucket4, .bucket5, .bucket6 {
}

Example 2: css multiple classes

//HTML Code
<div class = 'firstClass secondClass'></div>

//CSS Code
.firstClass.secondClass {
  
}

Example 3: css multiple classes

<div class="a-class another-class"></div>