_l_|--------------*------------* R
| |*** **
| | ** P **
_m_|--------------------***
| | *** |***
_k_|--------------***----+---** _r_
| _q_*| | ***
| *** | | ****
| *** | | ****
| *** | | ***
S |**** | | ****
| | | ** D'
| | |
| | |
| | |
| | |
0+-------------------------------------------------------- X'
N M.
Figure 1

In Fig. 1 the curve DD' represents the conditions of demand. It is
supposed to be drawn in such a way that if any point, Q, be taken on
the curve, and the perpendicular QN be drawn to meet the base line, or
axis OX, then ON will represent the amount that will be demanded at a
price represented by QN (or O_l_). In other words, distances measured
along OY represent prices, and distances measured along OX represent
quantities of the commodity, or service, or whatever it may be.
Clearly, then, the demand curve, DD', must slope downwards from left
to right, since the lower the price asked, the greater will be the
amount demanded. Similarly the curve SS' represents the conditions of