Thanks Partypooper. I've got a bit of good luck/will in reserve for the inevitable curse of the forum at the moment.

The amounts outlaid are for combining horses in all possible combinations i.e. 1 horse, 2 horses (doubles), 3 horses (trebles), 4 horses (quadrellas) and 5 horses (not sure what they're called - probably quintellas or something like that.

For my second stringers I'm coupling 4 horses this way so there are 15 combinations in which they can win and also place.

Call the imaginary races Race A, Race B, Race C and Race D. The possible combinations are:

1. a single bet on each horse. A B C D (4 bets in all)
2. all of the possible doubles AB, AC, AD, BC, BD, CD (6 bets in all)
3. all of the possible trebles ABC, ABD, ACD, BCD (4 bets in all)
4. all of the possible quadrellas ABCD (1 bet in all)

Altogether there are 15 combinations (for a $1 stake an outlay of $15).
With the dividends multiplying, if you are lucky enough to snag a few horses, the return can be sizeable.

For 5 horses there are 31 such combinations.

It's probably the horseplayers version of the lottery / windfall bet.

IASBET allow across code and different venue bets on this kind of bet which they list as 'multis'.

Hope this explains it a bit.