Though represented in English by two symbols this is a single sound, which may be either interdental or, as frequently in English, produced "by keeping the tongue loosely behind the upper front teeth, so that the breath escapes partly between the tongue and the teeth, and partly, if the teeth are not very closely set, through the interstices between them" (Jespersen).
The study of the spirants, c, 1, 1; g, j is made a very delicate one by the circumstance that the interdental pronunciation of c, 1 on the one hand, and the guttural pronunciation of g, j on the other, are of comparatively recent date, and convey no notion of the value of these letters before the 17th century.
The substitution of these interdental and guttural sounds for the surd and sonant spirants respectively did certainly not take place simultaneously, but the vacillations of the old orthography, and afterwards the decision of the Spanish Academy, which suppressed x (= I; x was retained for cs) and allows only c and g before e and i, I and j before a, a, a, make it impossible for us to follow, with the help of the written texts, the course of the transformation.
Some people find it easier to use interdental brushes rather than floss.
The six teeth are considerably worn down and have wide interdental notches; the points of the teeth form an irregular semicircle.