letter vs sigma

Other hyponyms of letter include ascender, descender, digram, digraph, initial, A, a, B, b, C, c, D, d, E, e, F, f, G, g, H, h, I, i, J, j, K, k, L, l, M, m, N, n, O, o, P, p, Q, q, R, r, S, s, T, t, U, u, V, v, double-u, W, w, ex, X, x, wye, Y, y, ezed, izzard, Z, z, zed, zee, alpha, beta, gamma, delta, epsilon, zeta, eta, theta, iota, kappa, lambda, mu, nu, xi, omicron, pi, rho, tau, upsilon, phi, chi, khi, psi, omega, aleph, beth, gimel, daleth, he, waw, zayin, heth, teth, yodh, kaph, lamedh, mem, nun, samekh, ayin, pe, sadhe, qoph, resh, sin, shin, taw, polyphone, polyphonic letter, block capital, block letter, vowel, consonant, yus.