The Ugaritic alphabet was one of the earliest Semitic abjads known. The city-state of Ugarit, on the north Syrian coast, used the script from at least the mid 2nd. millennium B.C.E until its destruction around 1190 B.C.E. Unlike all other northern Semitic alphabets, the Ugaritic alphabet represents almost all of the original 29 Semitic phonemes. The following diagram shows how the Ugaritic language had merged phonemes compared to the Ancient South Arabian languages (Sayhadic languages) which preserved all 29.
The only mergers to have occurred were between s¹ & s², and between ṣ & ḍ, both common mergers in the Canaanite languages, with which Ugaritic shares a close genetic relationship.
Something quite interesting about the Ugaritic alphabet is the order in which the letters were arranged. Two different orders have been found, one which matches quite closely to the Canaanite order, but with the extra letters inserted in certain places. This indicates that it's unlikely the Ugaritic alphabet order was borrowed from the Canaanite order, otherwise the extra letters would've been "tacked" on the end, and therefore it's likely the Canaanite order could've been borrowed from the Ugaritic. The other order that's been found matches almost exactly with the Musnad alphabet of the Sayhadic languages, but in this order a few letters are not matched with their correct counterpart and are instead "tacked" on the end. Again, this indicates it's unlikely the Musnad order was borrowed from the Ugaritic, but instead the Ugaritic order this time borrowed from the Musnad.
Interestingly in both orders, Ugaritic places the letter 'th' where we find the symbol for s¹ & s² in the Canaanite languages, whilst in the southern order it is where we find s² in the Sayhadic languages. Perhaps indicating a link between the sound value of s² in the Sayhadic languages, and the sound value of the merger between s¹ & s² in the Canaanite languages ('sh'). The symbol for s² in the Musnad alphabet also resembles the symbol for the merged phoneme s¹ & s² in the Canaanite languages, a "three-pronged" (or 'w') shape, as can be seen in the chart below.