Let z → θm(τ, z) denote the familiar theta function on Cg;; we recall that m = (m′m″) is in R2g and τ in 𝔖g. If m is in Q2g, we call θm(τ, 0) a theta constant. Since in this chapter we shall mainly consider theta constants rather than theta functions, we make an agreement that θm means the function τ → θm(τ, 0) on 𝔖g; accordingly, we shall write θm(τ, 0) = θm(τ). By an abuse of language, we call θm also a theta constant.