  We present calibrated photometric measurements of the earliest detections of
nearby type Ia supernovae (SNe Ia). The set of ~30 new, unfiltered CCD
observations delineate the early rise behavior of SNe Ia > 18 to 10 days before
maximum. Using simple empirical models, we demonstrate the strong correlation
between the risetime (i.e., the time between explosion and maximum), the
post-rise light-curve shape, and the peak luminosity. Using a variety of
light-curve shape methods, we find the risetime to B maximum for a SN Ia with
Delta m15(B)=1.1 mag and peak M_V=-19.45 mag to be 19.5+/-0.2 days. We find
that the peak brightness of SNe Ia is correlated with their risetime; SNe Ia
which are 0.10 mag brighter at peak in the B-band require 0.80+/-0.05 days
longer to reach maximum light.
  We determine the effects of several possible sources of systematic errors,
but none of these significantly impacts the inferred risetime. Constraints on
SN Ia progenitor systems and explosion models are derived from a comparison
between the observed and theoretical predictions of the risetime.
