Each of the twenty-four standard Tengwar of the Fëanorian alphabet was composed of two basic elements: a straight telco or stem, from which curved a rounded 'bow' shape known as a lúva. The nature of the bow defined the téma, or sound-series, to which a character belonged. Some curved upwards and leftwards from the stem, while others curved downwards and to the right. The lúva was sometimes 'closed' (with a horizontal bar connecting it back to the telco) or sometimes left 'open'.
For instance (using the conventions of the Third Age) all of the tengwar with an open lúva curving down and right belonged to the tincotéma, the series of 't' sounds, while all those with a closed, upward, left-curling lúva were part of the calmatéma (sounds related to 'k').
Though the direction and structure of the lúva was always constant within a series, it was often 'doubled' within that series (that is, a second bow would be drawn curving out from the first). For instance, the character tinco ('t') had a single lúva, while the 'voiced' version of the same sound, ando ('d') had a double bow.