The sidewall of a tyre shows a sequence of characters being the tyre size, e.g. 195/50 R16, where the first number is the cross section width measured between the tyre sides in millimetres.
The second number in tyre size is the height of profile (aspect ratio), i.e. the percentage to tyre width, e.g. 55 is 55% of 205, i.e. 112.75 mm. ”R” indicates the tyre type (R - radial, D - diagonal).
The last number denotes the width of a rim in inches. Knowing the tyre size is essential when making the purchase.