Most are classical Great refractors, which used achromatic doublets on an equatorial mount. The mupirocin pathway also contains several tandem acyl carrier protein doublets or triplets. An alternative to achromatic doublets is the use of diffractive optical elements. The dynein in the axoneme forms bridges between neighbouring microtubule doublets. When very short doublets were in fashion, codpieces were added to cover the front opening. The simplest theory is one with two Higgsinos and therefore two scalar Higgs doublets. There are also the additional fields and containing the electroweak Higgs doublets. This enables the measurements of quadrupolar doublets for each nonequivalent deuterium atom. The electroweak Higgs doublets come from an SO(10) 10. Since the two doublets can be identical this design is sometimes called a "symmetrical eyepiece". This has led to many cases of etymological doublets in these languages. Negative doublets, in which the negative-power element predominates, are also made. The theory plays further roles than just the explanations of doublets within electromagnetic spectrum. A good example consists of the doublets "frail" and "fragile". Examples of English-only doublets are "let and hindrance" and "have and hold".