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\subsubsection{The ${\rm Z \to \bbbar}$ partial width}  An electroweak correction of great interest is the vertex correction to the ${\rm Z} \to \bbbar$ partial width. This correction affects the total Z width $\Gamma_{\rm Z}$, the leptonic branching fraction $R_\ell$, the peak hadronic cross section $\sigma^{\rm peak}_{\rm had}$, and most sensitively, $R_{\rm b} \equiv \Gamma_{\rm Z \to \bbbar}/\Gamma_{\rm had}$. $R_b$ was measured at At  LEP and SLC SLC, $R_{\rm b}$ was measured  by tagging$b\bar{b}$ by  the presence of one tagged $b$-jet b-quark jet,  and the efficiency was controlled by double tag. the ``double tag'' method.  The present experimental value is $R_b value, $R_{\rm b}  = 0.21629 \pm 0.00066$ with 0.00066$, has  a roughly equal sharing between systematic and statistical errors. The uncertainties.     Because the  double $b$ tagging b-tagging  method is self-calibrating, and indeed most of the systematics are based on experimental tests related its accuracy is expected  to the modeling of events, and should decrease improve  with accumulated statistics. The SLD detector at SLC was had  the most efficient best efficiency for this selection,  by the double twofold  effect ofhaving  a more precise detector, granular vertex detector  and the a smaller  beam spot being smaller, thus allowing spot, which allowed  a more precise determination of the impact parameter of secondary hadrons. We expect While  the experimental conditions at TLEP are expected  to be similar to LEP with the exception that as those at LEP,  the beam spot  size at the IP is will be very significantly  smaller in all dimensions, ensuring that the $b$-tagging abilities dimensions than at SLC, and a next-generation vertex detector will be used. The b-tagging capabilities  should therefore  berather  similar to those of SLD or better  thanto  those of LEP. \\ SLD.\\  {\em A precision of $2-510^{-5}$ $2$ to $5 \times 10^{-5}$  seems therefore  to be a reasonable goal for the measurement of on $R_b$ $R_{\rm b}$  at TLEP} TLEP.}