Which sound is part of the English system but not a Spanish system?

The sound that is part of the English system but not the Spanish system is the "th" sound.

Here's why:

* English "th" sound: English has two "th" sounds: the voiceless "th" as in "thin" and the voiced "th" as in "this." These sounds are made by placing the tongue between the teeth and expelling air.

* Spanish doesn't have a "th" sound: Spanish uses other sounds to represent English "th" sounds. For example, "thin" might be represented by "cin" or "tin," and "this" might be represented by "dis" or "tiz."

Therefore, the "th" sound is a unique characteristic of the English sound system and does not exist in Spanish.