How does this Slope thing work?

Despite all the excellent communication from England Golf and other bodies golfers are still struggling to get their heads around the new World Handicap System. It's not the calculation of the Handicap that seems to be causing the most discussion but the differentiating factor between golf courses known as the 'Slope Rating'.

A few key facts are
Fact 1 – The lower your Handicap Index, the less the Slope Rating should concern you. A player with a zero Handicap Index plays off zero everywhere.
Fact 2 – The Slope Rating is a measure of the difficulty between different golf courses for higher handicap golfers.
Fact 3 – The Course Rating is a determination of the difficulty of a course against its par for scratch golfers.
Fact 4 – The Bogey Rating is a determination of the difficulty of a course against its par for the higher handicapper.
Fact 5 – There are several assumptions and criteria used in determining a Bogey Rating which naturally is going to lead to differences of opinion as to the definition of difficulty.
Set out below is a quick guide of the various terms using everyday language and providing a few illustrative examples.
How to Calculate your Handicap for a given course
Handicap = Handicap Index x (Slope Rating /113)
Slope Ratings differ by tees for courses and so a handicap may change depending on which tees are using.
Slope Rating
The Slope Rating for a golf course tells you how challenging the golf course is for a bogey player. The lower your handicap Index, the less relevance the Slope Rating is.
A player with a zero Handicap Index plays off zero at every course as going to back to our maths lessons zero multiplied by any number is zero.
There is a mathematical formula for calculating the slope which is
Slope = (Bogey Rating - Course Rating) x 5.381     (Ed – now you can forget that!)
By way of an example:
Wentworth West has a Slope Rating of 153 for the white tees. A player with a handicap index of 18.0 plays off a handicap of 24.
St. Andrews Old Course has a Slope Rating of 132 for the black tees. A player with a handicap index of 18.0 plays off a handicap of 21.
Handicap Index
The Handicap Index is the average of the best 8 Handicap differentials of your last 20 scores. The Handicap differential takes the performance versus the Course rating adjusted for the Slope Rating of the course in question, as follows:
Handicap differential = (Adjusted Gross Score - Course Rating) x 113 / Slope Rating
Course Rating
Course Rating is the average score a 'Scratch golfer' is expected to take on a course.
A 'Scratch golfer' is a player with a 'Handicap Index' of zero
By way of an example:
Wentworth West has a par of 73 off the white tees and the course rating is 76. A scratch player on average goes round in three over par.
St. Andrews Old Course has a par of 72 off the Black tees and the course rating is 73.1. A scratch player on average goes round in roughly one over par.
Bogey Rating
This is the average score a 'Bogey golfer' is expected to take on a course
A 'Bogey golfer' is a player with a 'Handicap Index' in the range 17.5 to 22.4
By way of an example:
Wentworth West has a par of 73 off the white tees and the bogey rating is 104.8. A bogey player on average goes round in thirty-two over par.
St. Andrews Old Course has a par of 72 off the Black tees and a bogey rating of 97.6. A bogey player on average goes round in twenty-six over par.
Determination of Course and Bogey Ratings
Course rating and the Bogey rating (+ slope ratings) usually change because of the effective playing length of the golf course. However, other factors go into the process including:

To a lesser extent, the obstacles of a course are taken into account in determining the ratings. Items such as the nearest boundary line, trees, hazard line, and whether there are any bunkers nearby are brought into the calculations.