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trigonalannal, channel, flannel, impanel, multichannel, panel •cracknel •grapnel, shrapnel •carnal •antennal, crenel, fennel, kennel •regnal •anal, decanal •adrenal, officinal, penal, renal, venal •signal, spignel •hymnal • cardinal • libidinal • ordinal •attitudinal, latitudinal, longitudinal •altitudinal •imaginal, paginal •marginal, submarginal •aboriginal • virginal • disciplinal •seminal •criminal, liminal, subliminal •abdominal, nominal, phenomenal, pronominal •noumenal •germinal, terminal •vaticinal, vicinal •sentinel • intestinal • Juvenaldoctrinal, final, semi-final, spinal, urinal, vaginal •quarterfinal •cantonal, O'Connell •cornel • nounal •atonal, Donal, hormonal, Monel, patronal, polytonal, tonal, zonal •motional •lagoonal, monsoonal, tribunal •communal •Chunnel, funnel, gunnel, gunwale, runnel, tunnel •autumnal • meridional •embryonal, Lionel •diagonal, heptagonal, hexagonal, octagonal, tetragonal •trigonal • orthogonal • occasional •divisional, provisional, visional •delusional, fusional, illusional •regional • original • coronal • arsenal •medicinal •impersonal, interpersonal, personal, transpersonal •irrational, national, passional, rational •factional, fractional, redactional, transactional •confessional, congressional, expressional, impressional, obsessional, processional, professional, progressional, recessional, secessional, sessional, successional •connectional, correctional, directional, interjectional, intersectional, sectional, unidirectional •ascensional, attentional, conventional, declensional, intentional, tensional, three-dimensional, two-dimensional •conceptional, exceptional, perceptional •durational, locational, oblational, relational, vocational •rotational •additional, positional, tuitional, volitional •fictional, jurisdictional •inscriptional • optional • proportional •devotional, emotional, notional, promotional •constitutional, evolutional, institutional, substitutional •constructional, fluxional, instructional •conjunctional, dysfunctional, functional, multifunctional •versional • seasonal •colonel, diurnal, eternal, external, fraternal, infernal, internal, journal, kernel, maternal, nocturnal, paternal, supernal, vernal

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trigonal (rhombohedral) One of the seven crystal systems (closely related to the hexagonal system) in which the unit cell has the shape of a rhombohedron, consisting of six crystal faces all of which have two pairs of parallel sides. The crystals may be referred to four crystallographic axes: one vertical axis of three-fold crystal symmetry, and three equal, horizontal axes of two-fold symmetry which are separated by an angle of 120°. The minerals calcite and quartz always occur in this system.