bode plot, zero at 0Hz I'm learning to draw magnitude bode plots from transfer functions:

$$ H(s) = \frac{sRC}{sRC+1} $$

So I can see that I have a zero at s = 0, which corresponds to 0 Hz. I know this will add a +20dB/dec slope for all frequencies in a logarithmic plot. I'm struggling to figure where the gain will start at the left side of my plot. I can kind of cheat a little bit because I know that this is a passive high pass filter, so I can start at the right side of the plot and call that 0dB, move to the left, and then apply my 20dB/dec slope once I pass the break frequency of the pole. But if the transfer function was more complicated and I didn't already know what to expect, what would I do?
$$H(s) = \frac{R C s}{R C s + 1}$$

If $R C s$ is much lower than 1 (i.e., $R C s \ll 1$, in Math), then you can make an approximation: $$H(s) \simeq \frac{R C s}{1}$$

Basically, the effect of the $R C s$ in the denominator becomes insignificant.

So, just pick a radian frequency well below $1 / R C$ and compute the magnitude of $H(s)$ at that point -- mark that on your plot, and draw your line rising at 20dB/decade from there.