Unfortunately it is not quite that simple!
Not all 700 tyres and rims are compatible
The best way of sizing tyres are what are called the European Tyre and Rim Technical Organisation (ERTRO) sizing
For many years there was a difference in tyre sizes, a 26"x1" tyre fits a rim with a diameter of 609 mm and a 26" x 2.125" tyre would fit a 673 mm rim
In oter words two 26" tyres, but with a difference in diameter of almost 3 inches!
The 700 series ae the same with a 700 x 18c and a 700 x 47 c having a difference of over 2 inches
So the idea was to have a tyre size that stated clearly the diameter of the tyre the format being xx-yyy
The important measurement is the three figure number as this is the diameter. It is the rim diameter, and if this number matches the rim then it will fit
The second number is the width of the tyre and can be varied
For instance on a fast commuter a "28" would work as a narrow fast tyre, and a "57" would be a wide tyre that would work better off road and offer more comfort