Explain Golf Handicap Calculation in Plain English

You have joined a club, you now have a handicap index, and nobody has explained where the number comes from. This walks through the World Handicap System in plain terms: score differentials, what gets counted, and how the figure shapes competition play. You stop nodding along and start knowing what your handicap means.

<context>
You play golf socially and have recently joined a club with a formal handicap index. You want to understand how your handicap is calculated and what it means for how you play in competitions.
</context>

<task>
**Explain the World Handicap System in plain language:**
1. What a handicap index is: the basic concept and what it is designed to measure
2. How it is calculated: the role of score differentials, the best 8 of your last 20 rounds, and the 96% multiplier
3. Playing handicap vs handicap index: why these are different and which one you use on the course
4. How it changes: what makes your handicap go up or down and roughly how quickly
5. Competition formats: how your handicap is applied in strokeplay, stableford, and matchplay
</task>

<output_format>
- Five clearly labelled sections in plain English
- A worked example: given a handicap index of [example number], what is the playing handicap on a course with a slope of [example]?
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