The second stroke of this character is a vertical stroke.
Tips
usage
竖 can be an adjective (vertical), a verb (to set upright), or a noun (vertical stroke in calligraphy). Opposite: 横 (héng, horizontal). Common phrase: 竖起大拇指 (give a thumbs up).
grammar
As a verb, 竖 often takes 起 as a complement: 竖起来 (stand something up), 竖起大拇指 (raise one's thumb). As an adjective, it modifies nouns directly: 竖线 (vertical line), 竖排 (vertical layout).
立 below is the indexing radical, a person planted upright on the ground. It anchors the meaning of 竖 as something standing vertically, and groups it with 站 (stand at attention), 端 (upright), and 章 (mark of standing rule).
刂 at upper-left is the knife radical in right-side form, repurposed here as a graphic stand-in for the older 臤 element. It serves a positional role on the page rather than carrying its blade meaning into the modern senses of upright and erect.
又 at upper-right pictures a right hand and contributes the action sense — a hand grasping or holding something up. Together with 刂 above, the upper portion is a simplified residue of the older 臤 (firm grip) element.
