user58279: My Arduino/PlatformIO libraries folder
libraries
└── cleaner_robot
    └── TB6612_Dual
        ├── tb6612_dual.h
        └── tb6612_dual.cpp

I used this code to innclude it:
#include <TB6612_Dual/tb6612_dual.h>

But the compilation has errors like "undefinded variables, undefinded reference,...etc" 
This is my sketch folder
Cleaner_Robot
└── src
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h

If i use this code, all is ok
#include "src/libs/TB6612_Dual/tb6612_dual.h"

So the question is how to include if header file is in a sub folder of a folder in libraries folder of Arduino/PlatformIO ?, i has searched for many things, but cant help. 
I also read about src folder allowed in Arduino, but where can find an official announcement/manual for this structure ?, cant find much about this
Thanks  :)   

user50084: I could include header files in subfolder after including root header file.
#include <TB6612_Dual.h>
#include <TB6612_Dual/tb6612_dual.h>

TB6612_Dual
└── src
    ├── TB6612_Dual.h << Add this empty file.
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h

Examples:

utils_asukiaaa/examples/string/string.ino
utils_asukiaaa/src/utils_asukiaaa/string.h
utils_asukiaaa/src/utils_asukiaaa/string.cpp
utils_asukiaaa/src/utils_asukiaaa.h


58279: <p>My Arduino/PlatformIO libraries folder</p>

<pre><code>libraries
└── cleaner_robot
    └── TB6612_Dual
        ├── tb6612_dual.h
        └── tb6612_dual.cpp
</code></pre>

<p>I used this code to innclude it:</p>

<pre><code>#include &lt;TB6612_Dual/tb6612_dual.h&gt;
</code></pre>

<p>But the compilation has errors like "undefinded variables, undefinded reference,...etc" </p>

<p>This is my sketch folder</p>

<pre><code>Cleaner_Robot
└── src
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h
</code></pre>

<p>If i use this code, all is ok</p>

<pre><code>#include "src/libs/TB6612_Dual/tb6612_dual.h"
</code></pre>

<p>So the question is how to include if header file is in a sub folder of a folder in libraries folder of Arduino/PlatformIO ?, i has searched for many things, but cant help. 
I also read about src folder allowed in Arduino, but where can find an official announcement/manual for this structure ?, cant find much about this</p>

<p>Thanks  :)   </p>
 50084: <p>I could include header files in subfolder after including root header file.</p>

<pre><code>#include &lt;TB6612_Dual.h&gt;
#include &lt;TB6612_Dual/tb6612_dual.h&gt;
</code></pre>

<pre><code>TB6612_Dual
└── src
    ├── TB6612_Dual.h &lt;&lt; Add this empty file.
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h
</code></pre>

<p>Examples:</p>

<ul>
<li><a href="https://github.com/asukiaaa/utils_asukiaaa/blob/d3e59913ba2bcb2f6866865791baa71592b01456/examples/string/string.ino#L1-L2" rel="nofollow noreferrer">utils_asukiaaa/examples/string/string.ino</a></li>
<li><a href="https://github.com/asukiaaa/utils_asukiaaa/blob/d3e59913ba2bcb2f6866865791baa71592b01456/src/utils_asukiaaa/string.h" rel="nofollow noreferrer">utils_asukiaaa/src/utils_asukiaaa/string.h</a></li>
<li><a href="https://github.com/asukiaaa/utils_asukiaaa/blob/d3e59913ba2bcb2f6866865791baa71592b01456/src/utils_asukiaaa/string.cpp" rel="nofollow noreferrer">utils_asukiaaa/src/utils_asukiaaa/string.cpp</a></li>
<li><a href="https://github.com/asukiaaa/utils_asukiaaa/blob/d3e59913ba2bcb2f6866865791baa71592b01456/src/utils_asukiaaa.h" rel="nofollow noreferrer">utils_asukiaaa/src/utils_asukiaaa.h</a></li>
</ul>
