Voltage difference between terminals of coil when current is changing is defined as:
U=L*di/dt

If the the rate of current change is high, you will have a very large voltage spikes that will sooner or later "fry" your transistor. Reverse biased diode is used to clamp these voltage spikes so that you transistor is safe and you don't have to use transistor with high voltage ratings. As an added protection I would add small value resistor between coil of relay and transistor.

Understanding of Relays and associated circuitry
Is there a popular Scrabble variation that: 


lets you play double/triple word scores twice, once vertically and 
once horizontally? 
does NOT let you play double/triple letter scores twice? 


This is how I've always played, so it surprised me to learn that 
official Scrabble rules do NOT permit using double/triple word scores 
twice. 

Perhaps this was true in an older version of Scrabble (I started 
playing in the 70s). 
