<html>
<body>
<div onmousedown="WhichButton(event)">Click this text with one of your mouse buttons to return a number.
<p>1 = The left mouse button</p>
<p>2 = The middle mouse button</p>
<p>3 = The right mouse button</p>
</div>
<p><strong>Note:</strong> The which property is not supported in IE8 and earlier versions.</p>
<script>
function WhichButton(event) {
alert("You pressed button: " + event.which)
}
</script>
</body>
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</html>