Tires: tire letter meaning, passenger car, horse trailer

Question
I have a horse trailer in need of new tires. The one's on it are: P235 R75/15 M+S. What does the "P" & the "M+S" mean? I also want to know "PLY" 6or8?, so i can get comparable tires. THANX

Answer
Sally,

The "P" stands for Passenger Car, and your vehicle is obviously NOT a passenger car.  You need a tire designed for trailer usage - an ST type tire or in a pinch an LT type tire.  Work with your local retailer because the load on the tire is so very important to its longevity - and obviously someone replaced the original tires with something else.  That makes it nearly impossible to figure out unless you have the thing in front of you.