subwoofer and normal speaker combo in same enclosure?

is it necessary for subwoofer to be in an individual enclosure ?

can't a normal speaker n subwoofer be in the same enclosure ?

what happens if they are in same enclosure ?
and what  "exceptional" happens if subwoofer is in individual enclosure ?

thank you SO much in advance !
i really wanna know this thing

They don't have to be but you get better overall sound quality if it is in a separate enclosure. If they are in the same enclosure the sound waves from the sub can overpower and cancel out those from the mid. Distorting your overall sound quality. You also gotta consider the volume of air the sub is able to move. This could force the cone of the mid past it's limits and damage the speaker. With the sub is a separate sealed off area of the enclosure and properly ported you'll get much better overall sound quality and kicking bass.