Korpa 
The Standard Series are made out of high-quality phenol resin (plastic) and feature a design based upon the top grade Custom Series. They are extremely consistent in specifications and playing quality, and offer excellent value for their price. The facing of this series is slightly shorter than that of the Custom Series making them easier to play, and easier for beginning students to achieve a rich sound.
These are the main differences between the Yamaha standard series clarinet mouthpieces:
3C: easy control and good response from lower to higher ranges, even for beginners.
4C: helps obtain a well-balanced tone over all octaves, for professionals and beginners alike.
5C: facilitates a richer, fuller volume than a 4C, with a greater tonal variety.
6C: an excellent mouthpiece for experienced players seeking powerful volume, strong tone and rich tone variety.
7C: a jazz favourite for penetrating volume with a rich, strong sound and precise tonality.