Concave Key / Key

When people put the key in the lock, they often confuse which side is right side so so they put the key after check with the eye or put the key carelessly once and remove, and finally put again to the right side. Eespecially at night, blind people and old mans often experience this problem.
every people's hand pose is same when they put the key. the thumb is placed at one side and other fingers is placed the other side.
The one side that the thumb should place is concave. so 'concave key' lead people to hold the key right side, and put in the lock.