I'm the person with the original question and thanks to several 
emails now understand that a standing martingale needs to go on a 
separate nose to avoid interfering with the 'action' of the 
Blair's...
People have responded with all sorts of excellent advice based on 
their personal interpretation of why I might be considering using a 
standing martingale...so I thought, to save people's (perhaps 
needless) typing, I would tell you *why* I asked.
Reason for hackamore: water squishes out of the side of Basil's mouth 
when he tries to drink in his normal bit (which he is happy in). He 
is a willing drinker and will attempt drinking whenever water is 
offered to him along the trail (and it breaks my heart to see him 
'lose' so much out of the side of his mouth, when he obviously wants 
to quench his thirst). So, I thought *if* we can travel along in a 
hackamore then he'd be able to drink freely whenever he wanted. In 
the UK, the Blair's is by far the most common configuration of 
bitless bridles we have, so that is the one that I bought (I have 
never seen a sidepull or bosal here, although I would be able to get 
one from *somewhere* if I *absolutely* wanted to).
When we travel along in his normal bit he is fine and I don't have 
any contact. I hoped we'd be able to do the same in the hackamore. 
The only times I have appreciable contact with the bit is when 
steadying him down rocky treacherous paths, or on the odd occassion 
where there is a bunch of people together on a narrow trail cantering 
fast in single file (as sometimes happens at Rides) when he will 
try to race, so I use my hands (and seat of course) a fair bit in 
those situations to avoid coming too close to the rear end of the 
horse in front. 
Now, we have been doing schooling (in an arena) with the hackamore 
and he is perfect and light and I don't need to use my hands (just 
my seat) to achieve downwards transitions. So, we have reached the 
stage where we have to go out on a hack to assess how the hackamore 
training is coming along. What is happening outside of the arena 
(perhaps not surprisingly, as arena work bores Basil, not me, so he 
is VERY happy to do a downwards transition in there!) is that, 
because he is more enthusiastic about his work, he evades 'steadying' 
commands (applied *gradually* in strength as I know that 
heavyhandedness with a mechanical hackamore is very severe on the 
nose) completely, until he reaches a point (when the command is 
increased in strength (seat and hands and voice)) where he raises his 
head (still not steadying his pace) until I have to disengage his 
hind end (terminology from Lucy Trumbull, cheers!) to stop him and 
then proceed again. Now in his bit he does *not* evade in this way 
and will go down rocky paths in a steady (and dare I say it?- 
collected) manner.
So I go back to the arena and do more schooling and change paces 
within gaits etc perfectly (argh!)....then I think, OK- we'll have 
to hit the trail again and see how it goes and so on...
He *is* improving without gadgets, but progress is slow (but then, he 
is a heavy-nosed, thickskinned pony and very strong in his halter so 
I'm not really surprised).
The trouble with the head-raising (while still moving) is that I view 
it as dangerous because we don't have any open ranges to ride on 
where we are in the UK and Basil is kept 6 miles from Leeds City 
Centre (second/third largest city in the UK) and our 'trails' all 
open up onto dual carriage ways (trunk roads) with heavy traffic (and 
because of work, I ride in the rush-hour frequently) and although he 
is traffic proof I don't want to end up bowling down a hill with his 
nose in the air and onto a busy road :-). As he is perfect in 
the the arena, it is very difficult to predict how he will be out on 
a hack, and I will only know how we are progressing by going out of 
the arena regularly.
So the standing martingale was intended to stop him raising his head 
(while we are practising in the hackamore) too high if he tried. I 
was not going to fix a 'hollow, resistant' pony because he isn't 
really (it is only in the situations I describe). But now, I don't 
want to start fiddling with a second noseband (and going out to buy 
the martingale as I haven't got one) so I think we'll either just 
persevere with the arena work and occassional 'tests' out and about, 
or perhaps give it up as a bad job and go back to our regular bit, as 
he is happy in that and does get *some* water down him in it (I'll 
just have to let him drink for a long time until he has managed to 
get his fill)..
Anyhow, sorry for the lengthy babble, but I just thought I ought to 
explain, before people put fingers to keyboard (perhaps 
needlessly....)
:-)
Cheers!
Liz and Basil (the nag)
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------------------------------------------------- E. Carpenter Institute for Cardiovascular Research The University of Leeds Leeds LS2 9JT, UK + 44 (0113) 2334173 e.carpenter@leeds.ac.uk -------------------------------------------------