Fly tying involves more that just applying a range of materials to a hook. Materials must be applied in a certain way depending on what kind of fly is to be created (nymph, dry fly or wet fly). Within fly tying there are some guiding principles that help the tier achieve the correct result.
These ensure that hackle, tail, wing and body have the correct dimensions, that nothing is too fat or thin or too long or short. To take note of these principles ensures the fly can be recreated many times over.

Proportion is more necessary with the tying of imitation patterns. Here the combination of body, tail and hackle must exactly match that of the corresponding insect. We are trying to fool the trout into believing the combination of fur, feather and synthetics are that of a real insect. Learning to correctly proportion flies takes time and experience, there will be times when the hackle may have been placed too far back or the head made too big etc. It is at times that the understanding of proportions comes into it's own. Thus follows a brief description of the correct proportions of three basic fly types:

1. Nymphs
   While nymphs can be tied to imitate many species of insect with many different body shapes most can be covered with one style tied in varying sizes.
   
   
   • The tail - Should be either one third or half the length of the hook shank.
   • The abdomen - Is usually two thirds of the hook shank and should taper toward the tail
   • The rib - Can vary from anything to 4 to 10 turns, typically the number of turns varies with the length of the hook shank (the longer the shank the more turns) so in this respect there is no hard and fast rule. When creating direct imitations however, the number of ribs should correspond with that of the chosen insect, the majority having ten.
   • The thorax - Should take up the remaining third of the hook shank and care should be taken not to finish the thorax to close to the hook eye, finish a couple of millimetres back to ensure room is left to remove excess materials and to whip finish without covering the hook eye.
   • Legs or hackle - The feather fibres or synthetic material imitating the nymph's legs should be the same length as the thorax.
   The head - Should be the same length as the hook eye.
   
   
2. Wet flies
   
   
   • The tail - As with nymph.
   • The body - Should extend from the back of the head to the point opposite the hook barb or where the shank seizes to be straight.
   • The rib - The standard is usually four or five but varies with the length of the hook shank (see nymph).
   • The wing - In general the wing should extend to just past the bend. However, many dressers opt for one and a quarter to one and a half times the length of the hook shank, both lengths are equally effective. The wing width should be three quarters of the hook gape.
   • The hackle - These fibres should be long enough to reach the hook point.
   • The head - As with nymph.
   
   
   
3. Dry flies
   Based primarily on those of the up winged family.
   
   
   • The wing - Is usually twice the length of the hook gape.
   • The tail - Should extend to either one or one and a half times the length of the body.
   • The body - Should extend from the base of the wing to a position opposite the hook barb, it should taper towards the tail and should be two-thirds the length of the hook shank.
   • The rib - Usually anything above four and under ten turns (See nymph).
   • The hackle - The length of the hackle fibre should be one and a half times the hook gape and should constitute the remaining third of the hook shank. After the hackle has been wound there must be enough room in front of the hackle to apply a head, this should be checked before the hackle is tied down and the stalk snipped off.
   • The head - As with nymph.