Syriac letters double as figures. The first nine letters carry one to nine, the next nine carry twenty to ninety, and the last four carry one hundred to four hundred. Kaph is where the second run begins, so it is the letter a scribe reaches for at twenty and the start of the whole middle stretch of any long number.
The system is inherited along with the alphabet and works the same way in Hebrew and, through the abjad order, in Arabic. Its practical use in Syriac manuscripts is thorough. Scribes number leaves and quires with it, which is how a codex that has lost its binding can be reassembled in the right order, and how a missing quire can be identified as missing rather than merely suspected. Dates, chapter numbers and marginal references use it too.
Figures beyond four hundred, which taw supplies, are written by combining letters or by marking a letter to multiply its value. A mark written below a letter can raise it in this way, so a scribe is not confined to what twenty-two letters can express on their own.
For a reader the system has one awkward property: a string of letters may be a word or a number, and the difference is signalled by marks that are not always present. In practice context settles it almost every time, and where it does not, an editor makes a choice and a careful edition says so.