Bootstrap: align input with button

Why don't buttons and inputs align well in Bootstrap?

I tried something simple like:

<input type="text"/><button class="btn">button</button>

The button is about 5px lower than the input in chrome/firefox.

Twitter Bootstrap 5

In Twitter Bootstrap 5, inputs and buttons can be aligned using the input-group class (see https://getbootstrap.com/docs/5.0/forms/input-group/#button-addons).

Group button on the left side

<div class="input-group mb-3">
  <button class="btn btn-outline-secondary" type="button" id="button-addon1">Button</button>
  <input type="text" class="form-control" placeholder="" aria-label="Example text with button addon" aria-describedby="button-addon1">
</div>

Group button on the right side

<div class="input-group mb-3">
  <input type="text" class="form-control" placeholder="Recipient's username" aria-label="Recipient's username" aria-describedby="button-addon2">
  <button class="btn btn-outline-secondary" type="button" id="button-addon2">Button</button>
</div>

Twitter Bootstrap 4

In Twitter Bootstrap 4, inputs and buttons can be aligned using the input-group-prepend and input-group-append classes (see https://getbootstrap.com/docs/4.0/components/input-group/#button-addons)

Group button on the left side (prepend)

<div class="input-group mb-3">
  <div class="input-group-prepend">
    <button class="btn btn-outline-secondary" type="button">Button</button>
  </div>
  <input type="text" class="form-control">
</div>

Group button on the right side (append)

<div class="input-group mb-3">
  <input type="text" class="form-control">
  <div class="input-group-append">
    <button class="btn btn-outline-secondary" type="button">Button</button>
  </div>
</div>

Twitter Bootstrap 3

As shown in the answer by @abimelex, inputs and buttons can be aligned by using the .input-group classes (see http://getbootstrap.com/components/#input-groups-buttons)

Group button on the left side

<div class="input-group">
  <span class="input-group-btn">
    <button class="btn btn-default" type="button">Go!</button>
  </span>
  <input type="text" class="form-control">
</div>

Group button on the right side

<div class="input-group">
   <input type="text" class="form-control">
   <span class="input-group-btn">
        <button class="btn btn-default" type="button">Go!</button>
   </span>
</div>

This solution has been added to keep my answer up to date, but please stick your up-vote on the answer provided by @abimelex.

---

Twitter Bootstrap 2

Bootstrap offers an .input-append class, which works as a wrapper element and corrects this for you:

<div class="input-append">
    <input name="search" id="search"/>
    <button class="btn">button</button>
</div>

As pointed out by @OleksiyKhilkevich in his answer, there is a second way to align input and button by using the .form-horizontal class:

<div class="form-horizontal">
    <input name="search" id="search"/>
    <button class="btn">button</button>
</div>

The Differences

The difference between these two classes is that .input-append will place the button up against the input element (so they look like they are attached), where .form-horizontal will place a space between them.

-- Note --

To allow the input and button elements to be next to each other without spacing, the font-size has been set to 0 in the .input-append class (this removes the white spacing between the inline-block elements). This may have an adverse effect on font-sizes in the input element if you want to override the defaults using em or % measurements.