I believe it came from the idea if you passed your enemy you'd be sword-hand to sword-hand (assuming your awfully polite enemy was keeping to his side of the road too and was also right-handed). If he was from France he'd be riding on the wrong side of the road though and all sorts of troubles would ensue, namely shouting about 'silly English k-niggets' and catupulting livestock at you.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys