Removing Double Vertices
This happens when two vertices share exactly the same point in space, and they are not connected. We need to remove double vertices as they can be the result of bad model editing or cause wrong interpretation of an external file. To remove double vertices (press W key and choose Remove Doubles or activate "Rem Double" option on the Mesh Tools menu) If any of the vertices are doubled, this tool will merge them into one vertex. In Mesh Tools menu, we also have an option called Limit, which defines the minimum distance that must exist between two vertices, for the Remove Doubles merges these vertices into one.
Extrude
This tool makes new geometry from selected vertices, edges and faces. click on E key after selection is made on the object and this can only be done in Edit mode. Three options are shown after press E key.
- Only Vertices: Extrude only vertex, result will be new vertices, and a line connecting these new vertices to the old ones
- Only Edges: Extrude only edges of the Mesh creating new planes. Only applies to 2 or more selected vertices.
- Region: Create the entire face, only available if we select a full face.
Extrude with Faces
When we select two faces, there will be two options for the extrusion.
- Region: This is the default extrude, where all faces are extruded in the same direction
- Individual Faces: This option extrudes selected faces along their individual normals, just as if they are extruded individually.
Constraining the Extrude
To constrain the extrusion along an axis, Press X, Y or Z Key immediately after pressing E Key.
Edge Length
To work with measurements, Select Edge Length on a menu call "Mesh Tools 1" or "Mesh Tools More"
Modifiers
There are several different types of modifiers. Each one can help us with specific problems, but there are some that are very useful for Architectural modeling. Such as Array, Subsurf, and Boonlean modifiers. (go to Modifier menu then Add Modifier)
Subsurf Modifier: This option helps to make model smoother. With more subdivisions, amodel that initially looks crispy with hard edges between the faces can gain soft edges between faces and look more organic. Heavily used in character modeling and more useful in furniture modeling and landscapes. 
- Level: controls the subdivision of the model for the 3D view.
- Render Level: controls the subdivision of the model for the rendering.
- Types of subdivisions: Simple Subdivision and Catmull-Clark, Catmull-Clark subdivision give us subdivision with smoothen edges between the faces.
If we want to control the level of subdivision and the way borders in different models work, we can use Loop Cut tool with the subdivide. A new loop cut near the edge of the model, will make the radius of the smoothening smaller, giving us more control over the details.
To make the model looks even softer in the 3D View and render, turn on a tool called "Set Smooth" located on the Link and Materials menu in the Editing panel. Ths option won't show any faces, only a smooth surface. "Set Solid" button to return to the surfaces to a faceted look.
- Array Modifier: A great tool for creating copies of objects, and organizing them into lines and columns. To use this modifier, we must select one object first, choose modifier either in Object or Edit Mode.
Choose between the three available methods:
1. Fit to Curve Length: Makes the copied objects follow a selected curve object. (e.g We can distribute trees or signs over a sidewalk.) Draw a curve over the surface, and apply the modifier to the objects that we want to copy to.
2. Fixed Length: Use a fixed length between the copied objects. (e.g Place copies of tree models in a straight line, and determine that between each copy, we will need a distance of 50 units)
Boolean Modifier: This modifier gives us a few extra options for creating or editing complex meshes. Such as Union, Subtraction and intersection. (however, it may result in triangular faces, making any post-editing will be difficult.)
Type of operations:
1. Difference: This option helps to subtract the shape of one object from anotehr. (e.g open a hole in a wall for a window. For this we must create the wall model and another object with the corresponding shape of the hole. Apply the modifier to create a new object based on the subtraction of the two shapes.)
2. Union: Create new object based on the union of two different meshes.
3. Intersection: Creates a new object based on the area where two different shapes intersect.
An example of an object with Boolean Modifier and Difference selected. The objective is to a hole on the door.To create the hole, we have to make 2 simple objects; Cube and Cylinder.
Select the rectangular object, and apply the modifier. Choose Difference, and type the name of the cylinder object. Click on Apply button to make a new object based on the difference. 
All boolean operations works the same way. Select one object, choose the operation, and type the name of the second object.
Mirror Modifier : Allow us to mirror one side of a model and make an inverted copy of the model. This modifier is simple and easy to use, however the only trick about it is that the center of the mirror must be set in the correct position. Else, the mirror will generate the copy at the wrong position.
Groups (CTRL + G)
Groups aren't a modeling tool, but they help to organize complex models. Several options will appear when CTRL+G is hit:-
Add to new group: creates a new group with the selcted objects.
Add to existing group: Another memu will pop-up this option is chose, it will allow us to choose in th group to which we want to add the objects.
Add to active object group: If we select more than one object, the last selected object will always be the active one. If this last object is already in a group, we will be able to add all of the previously-selected objects to the same group.
Remove from all group: Removes the object from the group.
When a group is created, we can identify an object in the group by the green line that marks this object when we select it. To view all groups available in a scene, go to the Object panel and choose the Object and Links menu. This will display all the grups available in the scene.
Proportional Editing (O key)
A tool that is very important in landscape modeling. It allows us to transform a vertex, and transfer part of the editing to the surrounding vertices.
In the header, we can find the selector fro different proportional editing types. This tool works together with the transformation options. We must select one or more vertices, and press G, R and S to begin. When the transformation is chosen, press the O key to turn proportional editing on. Along this option to turn proportional editing on and off, we cna choose from several types of proportional editing in the header.
If we use the 0 key to turn on the proportional editing, and then press G, a small circle will appear, surrounding the selected vertex. This is the area of the effect of the transformation, and all vertices inside this circle will be transformed proportionally. This is what happens if we proportionally transform this vertex in the Z-axis.
With the + and - keys on the numeric keyboard ot by using the mouse wheel, we can change the size of the circle.
To change the size of the preview, just click on the border of the menu and drag your mouse.
