I always believe that time should be taken in making the horse straight before bending in ernest commences. I was going through the processes of straightening and bending in order to give valued reason for my thoughts and the following is the result.
Firstly I read the relevant passages in a variety of books with regards to the bending / turning aids. I have quoted from a few below and then go through my thoughts.
1. "The Dressage Formula" - Erik Herbermann:
"Keep the horse tracking straight
Increase the weight of the inside seat bone - critical!
Passively and elastically resist on the inside rein
Play on the outside rein slightly without losing contact
Maintain an even rhythm and correct tempo, preventing the horse from running away from the bending influence (half halt)
Be sure the horses ears remain on a level
Do not overbend the horses neck
Only when the horse has released to the inside rein can the outside rein be judiciously brought into play to help further the bend more clearly through the horses body."
(NB - In my opinion the passive resist on the inside rein is to encourage the release through the jaw / poll and thus obtain individual influence over the respective hind leg. Without that release the horse will not step through with the inside hindleg and therefore will not execute correct bend).
2. "Misconceptions and Simple Truths in Dressage" - Dr HLM van Schaik :
To make a turn:
"It is very important that young horses learn to make correct use of their back. An inexperienced rider can make a lot of damage to the horse through misuse of hands in the turn."
"For the totally untrained horse - use an opening rein. The inside arm is moved as far as possible to the inside of the turn (ie similar to opening a door wide) and during this the finger nails should turn upwards, the reason being that there should be no downwards pressure on the bars of the jaw via the snaffle. In the beginning the outside hand should be totally passive. Inside leg with toes up so that calf muscle is tight presses just behind the calf thereby activating the inner hind leg. As soon as the horse has understood that he has to make a turn the outside hand, a little higher than the inside, pushes the outside rein against but never over the neck. That outside rein has to be taught enough so that it can function like a stick. In this way the horse learns to move the shoulders to the inside from the pressure of the outside rein, the indirect rein."
"Once this has been achieved and the horse realises a turn is being asked for this is the time to teach the horse the correct way, namely bending around the riders inside leg. The inside rein is now to be used to invite a slight flexion to the inside such that the rider can see the shimmer of the inside eye. The inside rein should be taught enough so that the vibration of the rein reaches the horses mouth, sometimes a gentle upward rotation of the inside hand is enough to get the flexion. The riders hand should stay close to the withers to prevent the head from coming in. By supporting the horse with the outside rein, pressing it close to the neck, asking for a change of direction and using the inside rein only to see the shimmer of the eye the rider will not impede the inside hind leg from making a larger stride which he would do if he pulled back on that rein. The goal is that the horse is made to make a larger stride with the inside hind as this will gymnastically develop the back muscles."
(NB - I personally do not teach the idea of having hands at different levels - it is not necessarily that I disagree with this text but in my opinion the novice rider having hands on differing levels leads to head tilting, crookedness and stiffness through the horse).
3. "Horses are made to be Horses" - Frainz Mairinger:
"Do not try to make a small turn until the horse can turn with ease on
one track. It is the same as a circle - do not try it until the horse
can go straight forward accepting the bit. If you do he will automatically
fight and resist. If you take the time to make the horse really go forward
to get him really straight and to get him really supple then you can start
to worry about other things."
"To get your horse on the lines and to make him straight is harder work and takes longer than to teach him all sideways movements, flying changes, piaffe, all movements. It is harder because it is the foundation."
"What is necessary for a turn?
It has been worked out that there are 3982 ways of riding through a turn wrongly. We have a large turn and to start off with the horse must be on one track. We have one leg following the other completely on one track, hind quarters not to either side but exactly following in the footsteps of the front legs. Once he is on the bit he maintains his rhythm and impulsion as he goes for the turn. He is flexed to the inside and the riders weight is evenly distributed - that is why he can do it. The riders inside leg is in absolute command of forward impulsion, the inside rein flexes with ease and he accepts it, the outside rein outside seat and outside leg control the hindquarters so that they do not fall away and are on the track. The inside leg drives him forward. He accepts your weight; therefore he does not try and rush, does not try and fall back, fall away, toss his head or anything else."
The first comment to make is that I totally appreciate that there will be numerous other texts that could lend for or against arguments to the bending / straightening discussion, but for the sake of not making this into a book I picked these three. What I would initially say is that these three quotes are from eminent classical trainers and riders, but all three show slight variances to their methodology. The use of the inside seat bone, the raising of the inside hand etc. goes to show that you will never have one definitive answer.
To my first thought. Erik Herbermann also states that the gaits should be pure while executing bend. This makes logical sense. Following this through, for the gaits to be pure the horse must be straight. The conclusion being that the horse must be straight on straight lines before bending should be asked for, otherwise you will asking for bend and obtaining gait purity at the same time!
This led me to my next thought: What is the definition of a horse being straight?
The normal answer is that the right hind follows the right fore and left hind to left fore. But doesn't this apply to bending as well? Doesn't the right hind follow the right fore through a bend? Of course it should, so therefore we need to clarify more. For me a horse will be straight if it tracks true (hind following fore on each side), takes equal weight on both hindlegs (not necessarily engaged yet - simply both hind legs taking equal responsibility) and through this has the ability to respond equally to the riders aids, no matter which direction he is being ridden in.
In my opinion there needs to be a sequence of events before bending proper takes place. The horse needs to be even on both sides, stepping equally into both hands and responding equally to the aids from either side. We know that the horse will have a contracted (hollow) side and an elongated (stiff) side. We want the contact even but initially the horse will not take up an even contact. We should never pull back so the only option left is to expand the contracted side out to match the other. The first priority therefore is that the horse should seek the contact. For this to happen the horse must be loose over the topline - longitudinally supple. Then the horse should be ridden forwards, out and straight such that the horse seeks the hand, but evenly into both hands. This will involve both hind legs doing the same job. This may involve a little positioning or counter positioning but the riders main focus is to ride the horse out to the hand. Only then, with the horse stretching over its topline to the hand and being actively sent forwards is the rider in a position to utilise the rein aids effectively for other things, namely stopping and turning. We are always reminded of needing co-ordination when we ride, the utilisation of both reins, not one, but for this to truely happen the horse must be into both otherwise the rider will not achieve this co-ordination. It will be more difficult to achieve on the correction horse as opposed to the young horse but the principle is still the same.
The rider therefore rides the horse straight and thus ends up with equal weight into both hands (ie the horse is pushing equally with both hind legs). The use of turns, then bending, then circles leading to lateral work is to increase the carrying capacity of each hind leg building towards collection, but each leg is increased at the same time, otherwise you lead back to crookedness.
To start to work on bending without the horse truly straight I believe will lead to increased crookedness. I have always believed in asking for the same work on both reins, ie if I want a 15m circle, the horse should be asked this in both directions. This is because I wish the benefit of a 15 m circle to be the same on each individual inside hind leg. If the horse could not be ridden with equal weight on both hinds in the straight line then when you ride these 15m circles there is uneveness at the outset which will be confirmed through the incorrect bending work.
I will leave you with one more small exert from Mairinger:
"The line principle - and you have to try and understand it - is that every line you put him on, and try to keep him on, is a gymnastic exercise for him. Do not be surprised if it takes a bit of doing."