#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> PII;
#define Yes cout<<"YES\n"
#define No  cout<<"NO\n"
#define pb push_back
#define endl '\n'
int n,m,k;
const int N=1e3+10;
int a[N][N],vis[N][N];
int num;
void init(){
    int tmp=1;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            a[i][j]=tmp;
            if(tmp) num++;
            tmp^=1;
            if(a[i][j]){
                if(i==1 || i==n){
                    if(j-1 && n-j) vis[i][j]=3;
                }
                else if(j==1 || j==n){
                    if(i-1 && n-i) vis[i][j]=3;
                }
                else vis[i][j]=4;
            }
        }
        if(n&1);
        else tmp^=1;
    }
    vis[1][1]=vis[1][n]=vis[n][1]=vis[n][n]=2;
    // for(int i=1;i<=n;i++){
    //             for(int j=1;j<=n;j++) cout<<(a[i][j]?"R ":"B ");cout<<endl;}
}
bool solve () {
    for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) vis[i][j]=0;
    cin>>n>>k;
    num=0;
    int tmp=2*(n-1)*n;
    if (k==1 || k==tmp-1){
        cout<<"Impossible\n";
        return 0;
    }
    if(k==0){
        cout<<"Possible\n";
        for(int i=1;i<=n;i++){
            for(int j=1;j<=n;j++){
                cout<<"R";
            }
            cout<<"\n";
        }
        return 0;
    }
    if(n==3){
        cout<<"Possible\n";
        if (k == 0) cout<<"RRR\nRRR\nRRR\n";
        else if (k == 2) cout<<"BRR\nRRR\nRRR\n";
        else if (k == 3) cout<<"RBR\nRRR\nRRR\n";
        else if (k == 4) cout<<"RRR\nRBR\nRRR\n";
        else if (k == 5) cout<<"BRR\nRRB\nRRR\n";
        else if (k == 6) cout<<"RRR\nBRB\nRRR\n";
        else if (k == 7) cout<<"RRR\nBRB\nRBB\n";
        else if (k == 8) cout<<"RBR\nBBB\nRBR\n";
        else if (k == 9) cout<<"RRR\nBRB\nRBR\n";
        else if (k == 10) cout<<"BBR\nBRB\nRBR\n";
        else if (k == 12) cout<<"RBR\nBRB\nRBR\n";
        return 0;
    }
    init();
    if(k==tmp) return 1;
    bool flag=0;
    int x=tmp-k;
    // cout<<tmp<<" "<<x<<endl;
    const int n2=4/((n&1)?1:2);
    int n3;if(n&1){n3=4*((n+1)/2-2);}else{n3=4*(n/2-1);}
    const int n4=num-n2-n3;
    // cout<<n2<<" "<<n3<<" "<<n4<<endl;
    int c4=x/4,c3,c2,c1;
    // cout<<c4<<"__\n";
    if(c4<=n4){
        int t=x%4;
        // cout<<t<<endl;
        for(int i=1;i<=n;i++){
            for(int j=1;j<=n;j++)
                if(vis[i][j]==4)
                        if(c4){a[i][j]^=1;c4--;}
        }
        if(t==3) a[1][3]^=1;
        else if(t==2) a[1][1]^=1;
        else if(t==1){
            a[1][3]^=1;a[1][1]^=1;
            a[2][2]^=1;
        }
        return 1;
    }
    else{
        c3=(x-n4*4)/3;
        // cout<<c3<<"==\n";
        if(c3<=n3){
            int t=(x-n4*4)%3;
            // cout<<"____"<<t<<endl;
            for(int i=1;i<=n;i++){
                for(int j=1;j<=n;j++)
                    if(vis[i][j]==4) a[i][j]^=1;
                    else{
                        if(vis[i][j]==3){
                            if(c3){a[i][j]^=1;c3--;}
                        }
                    }
            }
            if(t==2) a[1][1]^=1;
            else if(t==1){
                a[n][n]^=1;
                a[1][1]^=1;
                a[1][3]^=1;
            }
        }
        else{
            c2=(x-4*n4-3*n3)/2;
            for(int i=1;i<=n;i++){
                for(int j=1;j<=n;j++)
                if(vis[i][j]>=3) a[i][j]^=1;
                else{
                    if(vis[i][j]==2){
                        if(c2){a[i][j]^=1;c2--;}
                    }
                }
            }

            // for(int i=1;i<=n;i++){
                // for(int j=1;j<=n;j++) cout<<(a[i][j]?"R":"B");cout<<endl;}

            int t=(x-n4*4-n3*3)%2;
            if(t==1){
                if(n&1){
                    a[1][3]^=1;
                    a[n][n]^=1;a[n][1]^=1;
                }
                else{
                    a[1][1]^=1;
                    a[n][n]^=1;
                    a[1][3]^=1;
                }
            }
        }
        return 1;
    }

}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int T;
    cin>>T; while (T--){
        if(solve()){
            cout<<"Possible\n";
            for(int i=1;i<=n;i++){
                for(int j=1;j<=n;j++) cout<<(a[i][j]?"R":"B");cout<<endl;}
        } 
        // else cout<<"Impossible\n";
    }
    return 0;
}