Hammer Vertex Tool

Vertex manipulation gives you complete control over the shape of a solid object. Using the Vertex Tool, you can reshape the object by manipulating individual vertices and brush edges, easily creating odd (and sometimes invalid) shapes. This is useful for modifying specific pieces of a group of objects, where resizing would interfere with other objects in the group. Also, it allows you to make shapes that are impossible with Plane Clipping, and extremely difficult to do with carving.
== Vertices and edges ==
A vertex (or plural, vertices) is a corner of a solid. Moving one vertex will not affect any other vertex, but it will change the position of any edges associated with it. An edge is a point between two vertices which, when moved, will change the position of its two associated vertices, as well as their associated edges (no other vertices are affected besides the first two).
While in vertex edit mode, you can select and move vertices, much like using the Selection Tool. Drag a selection box around vertices, and press to select them. Hold down and click individual vertices to add or remove them from the selection. You can select different objects to vertex edit by clicking the "x" handle in the center of the object. You can also select multiple objects at once to perform vertex editing upon by holding the key.
To edit the vertices of an object:
# Select the object(s) you would like to use vertex manipulation on.
# Select the Vertex Tool from the Map Tools Toolbar. The selected solid will turn into a wireframe image with partial shading. Vertices will be marked as white dots, and edges as yellow dots.
# Click on the vertices/edges you want to move. They will be highlighted in red. Note that you can move both vertices and edges.
# You can either drag the selected vertices or edges with the mouse, or move them in the selected 2D view using the arrow keys.
== Face splitting and removal==
The Vertex Tool also allows you to split the faces on an object, adding additional faces to the brush. Simply select two opposing edges and press . This will turn the two edges into vertices, and place a new edge between them. Here is an example of face splitting:
It is also possible to merge vertices, doing so you can remove faces from an object. To do so simply drag any number of vertices onto exactly the same point as other vertices on the same edge. Once the vertices overlap a dialog should appear asking if you wish to merge the vertices that overlap. Press yes and the vertices will now count as one.
You can also split a face into two triangles diagonally by selecting two opposite corners and pressing . This is useful for creating spiral ramps (or clip brushes for spiral staircases) without having to slice each entire brush in half.
===Only Split Once===
Splitting a face multiple times (splitting a face that was generated after splitting another face) usually results in invalid solids being created. The solid appears correctly in the orthographic 2D views, but in 3D view a plane with 0 thickness is visible (highlighted in grey). This was created after the front face was split in half, followed by splitting the right half of the resulting face again. 
This is an invalid solid, and while your map will compile with this error it will not run. This error is fairly easy to avoid. If you need to create complex world geometry try to use multiple brushes instead of multiple face splits to create the shapes. This can generally be remedied by splitting the offending faces diagonally into triangles; however, this can also cause the solid to become concave, which is also not allowed. If this happens, undo the split and try splitting it between the other two corners.
== Vertex scaling ==
You can also perform vertex scaling with the Vertex Tool. Select the vertices you wish to scale and press . The Scale dialog will open, and a circle with a dot in the center will appear in the center of the selected vertices.
The circle is used as the center of the scale operation. Drag the circle in any of the 2D views to reposition it. Pressing the arrows in the Scale dialog box will increase or decrease the amount of scaling done on the selected vertices. Press when you are done to close the Scale dialog box and return to normal vertex edit mode.
To scale a set a vertices on an object:
# Select the object(s) you would like to scale vertices on.
# Select the Vertex Tool from the MapTools Toolbar. The selected solid will turn into a wireframe image with partial shading. Vertices will be marked as white dots, and edges as yellow dots.
# Click on the vertices/edges you want to scale. They will be highlighted in red.
# Press to enter to bring up the Scale dialog box.
# Alter the center of the scale operation my clicking and dragging the white circle in the center of the vertex selection.
# Change the Scale value to alter the selected points. You can press the up or down arrows to move in 0.1 increments, or enter a scale value directly in the text area. The scale changes will automatically be reflected on the selected object.
# Press or when you are done scaling to close the Scale dialog box.
== Vertex Snap to Grid ==
In very select few versions of Hammer (atleast in all Source 2013 games and 2013 SDK Base since May 2014 update), you can snap the verts of a brush to the nearest grid point with 
Category:Hammer Tools